busybox: remove busybox menuconfig from top level menuconfig

This will help with avoiding bogus bug reports caused by stale configs
and bad cases of user tinkering :)

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39106 3c298f89-4303-0410-b956-a3cf2f4a3e73
busybox: add support for config overrides via env/busybox-config

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39107 3c298f89-4303-0410-b956-a3cf2f4a3e73