nativesdk was just unsetting LIBCOVERRIDE however that causes some build failures
for xorg-libs which used a libc override. This adds in a mechanism to force
nativesdk to glibc and give the option of allowing another selection like
uclibc if anyone ever does the work to make it operational.
(From OE-Core rev: 154f5782d95b4bca8e574d3d3fde170ce3d196ae)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>