bb-imager: Use rpi config format

- Allow having submenu when selecting image (not implemented in GUI yet)
  similar to rpi-imager allowing bootloader updates from the rpi-imager
  now.
- Some new fields like Flasher have been added, but I have made sd card
  flashing the default, so the format is completely backward compatible.
- The config parsing also ignores any unsupported flasher types, making
  the config pretty much forward compatible as well.

Changelog: changed
Signed-off-by: Ayush Singh <ayush@beagleboard.org>