With several subtargets, the image/Makefile becomes crowded after a
while. Many targets have moved their device definitions to $subtarget.mk
files to have them more organized, let's do this here as well.
Cc: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
