busybox: Add help documentation in menuconfig

Explaining why you shouldn't be here, and about version tainting.

Signed-off-by: Karl Palsson <karlp@remake.is>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39440 3c298f89-4303-0410-b956-a3cf2f4a3e73