busybox: add back support for top-level menuconfig based config overrides (can be used by custom packages to alter the config)

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39125 3c298f89-4303-0410-b956-a3cf2f4a3e73