kernel: add a new global config symbol for enabling rfkill support (can be enabled by default via target feature flag)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37491 3c298f89-4303-0410-b956-a3cf2f4a3e73