iop: genconfig: Get subtarget using data from OpenWrt

Determine the subtarget for a given device using data generated from the OpenWrt build system instead of parsing the image makefiles directly. This provides correct results even when fancy tricks like includes inside the Makefiles are used, which is the case for an upcoming feature where additional devices can be defined in external feeds.

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