buildroot

Reset critical environment variables

Reset them early in top-level Makefile to ensure they're keept unset
all the way through the build process.

This also removes the requirement of these variables being unset
globally prior the build.

Signed-off-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>