MAKEALL: find kernel headers better
MAKEALL: find kernel headers better

Work with pre-sysroot compilers that have kernel headers available in the
standard location, and let people force the location manually.

Signed-off-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@gentoo.org>

joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc