mel: unexport some TARGET_ variables

This prevents TARGET_{C,CPP,CXX,LD}FLAGS from leaking into native checksums,
causing native sstates to not be shared between machines where those flags
differ. Discovered by Mark Crowe, see the oe-core list for details.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
mel: clean up, drop a few old now-unnecessary bits

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #133 from kergoth/sstate-reuse-fix

mel: unexport some TARGET_ variables