kirkwood: exploit BOARD_NAME to set DEVICE_DTS and SUPPORTED_DEVICES

kirkwood: exploit BOARD_NAME to set DEVICE_DTS and SUPPORTED_DEVICES

Due to the history of the target, all devices added before a certain
point have the same device string in BOARD_NAME, DEVICE_DTS and added
to SUPPORTED_DEVICES.

Thus, we can set this one automatically for all devices where
BOARD_NAME is specified, removing the explicit DEVICE_DTS and
SUPPORTED_DEVICES addition there.

For new devices, nothing has changed, and just DEVICE_DTS has to
be set manually.

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