ar71xx: reorganize kernel configuration

ar71xx: reorganize kernel configuration

Rather than including all machines for the generic subtarget in the base
configuration, leave the base configuration without specific machines and
create a proper subtarget config for ar71xx/generic. The configuration
diffs of the mikrotik and nand subtargets get significantly shorter, as
will the configurations of future subtargets.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>