Support subtarget in buildscript

Prepares a subtarget variable for cases where it is not 'generic'

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
- Rebased
Signed-off-by: Tim Niemeyer <tim@tn-x.org>