Draft: Add support for Apple iPhone 7 flasher

This merge request adds support to flash Apple devices (especially the iPhone 7/7+) using checkra1n and pongoOS Linux loader script.

To get this working we would need:

• checkra1n packaged, which exploits the Apple device and starts a custom bootloader called pongoOS
• checkra1n is not open sourced yet so it is possibly that it won't get packaged into Alpines aports. Is it possibly to have this im pmaports?
• Adding flash method into deviceinfo file and add custom dtb and compressed kernel image, see MR here
• After pongoOS is bootet, an other script will wait until it's ready, flashes and boots the Linux kernel
• Both tools will require root access to the phone. This would be easy with sudo but maybe this is not prefered in pmbootstrap. I tried udev rules for non-root access but didn't had success yet.

Manually running these steps already works and it would be cool to have them into one single pmbootstrap flasher boot command to simplify the process. I still have to test this implementation :)

pmb/config/__init__.py

@@ -652,7 +652,7 @@ install_user_groups = ["wheel", "video", "audio", "input", "plugdev", "netdev"]
 # FLASH
 #
 
-flash_methods = ["fastboot", "heimdall", "0xffff", "uuu", "none"]
+flash_methods = ["fastboot", "heimdall", "0xffff", "uuu", "pongoos", "none"]
 
 # These folders will be mounted at the same location into the native
 # chroot, before the flash programs get started.
@@ -779,6 +779,16 @@ flashers = {
                 ["uuu", "flash_script.lst"],
             ],
         },
+    },
+    "pongoos": {
+        "depends": ["pongoos-loader", "checkra1n"],
+        "actions": {
+            "boot": [
+                # Following commands require root access to run successfully
+                ["sudo", "sh", "-c 'checkra1n -cpE && \
+                        pongoos-loader -k $BOOT/Image.lzma -d $BOOT/dtbpack'"]
+            ],
+        }
     }
 }