brcm47xx: move current target to "generic" subtarget

This allows creating more subtargets and optimize builds per family.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41024 3c298f89-4303-0410-b956-a3cf2f4a3e73