busybox: add a reworked implementation of menuconfig support, this time with a guard option that keeps all symbols at default values until an extra option is activated

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39435 3c298f89-4303-0410-b956-a3cf2f4a3e73