Add "fastboot-bootpart" flasher to flash split images using Fastboot

asus-me176c has a Fastboot interface that can be used for flashing, but in postmarketOS we do not use Android boot images for it. This is because is it not very practical - the boot partition is quite small and there is a (custom) EFI bootloader that can boot directly from any other FAT32 partition.

At the moment the installation process is manual:

1. pmbootstrap install --split to have separated boot (FAT32) and rootfs images
2. pmbootstrap export
3. Flash boot and rootfs images manually using Fastboot

The "fastboot-bootpart" flasher implements that process in a more convenient way. When a device uses the "fastboot-bootpart" flasher:

• We generate --split images on "pmbootstrap install" by default. (This can be disabled using --no-split instead.)
• pmbootstrap flasher flash_kernel flashes the raw boot partition (not an Android boot image) using Fastboot, just like the rootfs.

There are some limitations that could be improved in the future:

• "fastboot-bootpart" is not offered in the device wizard. I think it is special enough that no-one will be starting with it, and the difference to normal "fastboot" might be confusing.

• Support "pmbootstrap flasher boot". asus-me176c does not support "fastboot boot" properly, but theoretically we could still generate Android boot images to use when booting an image directly.

• At the moment the boot partition image is not regenerated when using "pmbootstrap flasher flash_kernel" (unlike when using Android boot images). "pmbootstrap install" needs to be run manually first.

I think @Mis012 was eventually interested in using something like this together with his "re-boot" bootloader for MSM8916.

pmb/config/__init__.py

@@ -344,7 +344,9 @@ Flasher abstraction. Allowed variables:
 
 $BOOT: Path to the /boot partition
 $FLAVOR: Kernel flavor
-$IMAGE: Path to the rootfs image
+$IMAGE: Path to the combined boot/rootfs image
+$IMAGE_SPLIT_BOOT: Path to the (split) boot image
+$IMAGE_SPLIT_ROOT: Path to the (split) rootfs image
 $PARTITION_KERNEL: Partition to flash the kernel/boot.img to
 $PARTITION_SYSTEM: Partition to flash the rootfs to
 
@@ -355,15 +357,31 @@ uuu specific: $UUU_SCRIPT
 flashers = {
     "fastboot": {
         "depends": ["android-tools"],
-        "actions":
-                {
-                    "list_devices": [["fastboot", "devices", "-l"]],
-                    "flash_rootfs": [["fastboot", "flash", "$PARTITION_SYSTEM",
-                                      "$IMAGE"]],
-                    "flash_kernel": [["fastboot", "flash", "$PARTITION_KERNEL",
-                                      "$BOOT/boot.img-$FLAVOR"]],
-                    "boot": [["fastboot", "--cmdline", "$KERNEL_CMDLINE",
-                              "boot", "$BOOT/boot.img-$FLAVOR"]],
+        "actions": {
+            "list_devices": [["fastboot", "devices", "-l"]],
+            "flash_rootfs": [["fastboot", "flash", "$PARTITION_SYSTEM",
+                              "$IMAGE"]],
+            "flash_kernel": [["fastboot", "flash", "$PARTITION_KERNEL",
+                              "$BOOT/boot.img-$FLAVOR"]],
+            "boot": [["fastboot", "--cmdline", "$KERNEL_CMDLINE",
+                      "boot", "$BOOT/boot.img-$FLAVOR"]],
+        },
+    },
+    # Some devices provide Fastboot but using Android boot images is not practical
+    # for them (e.g. because they support booting from FAT32 partitions directly
+    # and/or the Android boot partition is too small). This can be implemented
+    # using --split (separate image files for boot and rootfs).
+    # This flasher allows flashing the split image files using Fastboot.
+    "fastboot-bootpart": {
+        "split": True,
+        "depends": ["android-tools"],
+        "actions": {
+            "list_devices": [["fastboot", "devices", "-l"]],
+            "flash_rootfs": [["fastboot", "flash", "$PARTITION_SYSTEM",
+                              "$IMAGE_SPLIT_ROOT"]],
+            "flash_kernel": [["fastboot", "flash", "$PARTITION_KERNEL",
+                              "$IMAGE_SPLIT_BOOT"]],
+            # TODO: Add support for boot
         },
     },
     # Some Samsung devices need the initramfs to be baked into the kernel (e.g.
@@ -373,8 +391,7 @@ flashers = {
     # "isorec" (isolated recovery), a term coined by Lanchon.
     "heimdall-isorec": {
         "depends": ["heimdall"],
-        "actions":
-        {
+        "actions": {
             "list_devices": [["heimdall", "detect"]],
             "flash_rootfs": [
                 ["heimdall_wait_for_device.sh"],
@@ -388,8 +405,7 @@ flashers = {
     # fastboot compatible devices. Example: s7562, n7100
     "heimdall-bootimg": {
         "depends": ["heimdall"],
-        "actions":
-        {
+        "actions": {
             "list_devices": [["heimdall", "detect"]],
             "flash_rootfs": [
                 ["heimdall_wait_for_device.sh"],
@@ -400,20 +416,18 @@ flashers = {
         },
     },
     "adb": {
-            "depends": ["android-tools"],
-            "actions":
-            {
-                "list_devices": [["adb", "-P", "5038", "devices"]],
-                "sideload": [["echo", "< wait for any device >"],
-                             ["adb", "-P", "5038", "wait-for-usb-sideload"],
-                             ["adb", "-P", "5038", "sideload",
-                              "$RECOVERY_ZIP"]],
-            }
+        "depends": ["android-tools"],
+        "actions": {
+            "list_devices": [["adb", "-P", "5038", "devices"]],
+            "sideload": [["echo", "< wait for any device >"],
+                         ["adb", "-P", "5038", "wait-for-usb-sideload"],
+                         ["adb", "-P", "5038", "sideload",
+                          "$RECOVERY_ZIP"]],
+        }
     },
     "uuu": {
         "depends": ["uuu"],
-        "actions":
-        {
+        "actions": {
             "flash_rootfs": [
                 # There's a bug(?) in uuu where it clobbers the path in the cmd
                 # script if the script is not in pwd...