target: feed-device: Introduce feed-devices mechanism

Introduce a mechanism where image Makefiles (target/linux/*/image/Makefile) can optionally include target/linux/feed-devices/Makefile to enable consuming additional device profiles that are defined in other feeds. The use case for this is to have some devices stored in a different repo with a different access level.

The Makefile includes a list of direcotries feed-devices-list.mk which right now is created by ./iop feeds_update. The functionality might later be moved to scripts/feeds. This list will be used to determine the directories to include extra devices from.

The directory structure in those devices mirrors the directory structure in the target directories, i.e. (BOARD)/image/(SUBTARGET).mk.

Signed-off-by: Andreas Gnau andreas.gnau@iopsys.eu