Initial PINE A64-LTS support

Here's my WIP branch for the PINE A64-LTS, mainline u-boot crashes before starting the kernel:

U-Boot SPL 2018.05 (Sep 08 2018 - 15:35:14 +0000)
DRAM: 0 MiB
### ERROR ### Please RESET the board ###

This branch also depends on https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1693

device/device-pine-a64lts/deviceinfo

@@ -21,4 +21,4 @@ deviceinfo_dev_keyboard=""
 # Bootloader related
 deviceinfo_flash_method="none"
 deviceinfo_generate_legacy_uboot_initfs="true"
-deviceinfo_write_uboot_spl="/usr/share/u-boot/pine64_plus/u-boot-sunxi-with-spl.bin"
+deviceinfo_write_uboot_spl="pine64_plus/u-boot-sunxi-with-spl.bin"