at91: introduce vendor_model scheme and drop board names

at91: introduce vendor_model scheme and drop board names

This introduces the vendor_model scheme to this target in order to
harmonize device names within the target and with the rest of
OpenWrt. In addition, custom board names are dropped in favor
of the generic script which takes the compatible.

Use the SUPPORTED_DEVICES variable to store the compatible where it
deviates from the device name, so we can use it in build recipes.

While at it, harmonize a few indents as well.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>