[buildroot] add preliminary support for musl

Musl is an alternative C-library, see http://www.musl-libc.org

 for more infos.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34314 3c298f89-4303-0410-b956-a3cf2f4a3e73