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.
• Fixes #34 (closed) , #42 (closed)

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