buildroot: make it easier to build all kmods

Split out kmods from ALL to make it easier to create local builds that
are compatible kmod-wise with releases.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44830 3c298f89-4303-0410-b956-a3cf2f4a3e73