install: add flash_boot for fastboot-bootpart
When using fastboot-bootpart flash method to split root/boot partitions out, it was no longer possible to flash the Android boot partition.
Add a new flash_boot action to handle this.
This will allow us to migrate devices like the OnePlus 6 to have the boot partition (ESP) on system, and the rootfs on userdata rather than the current subpartition method.
Signed-off-by: Caleb Connolly caleb@postmarketos.org