Makefile: Tone down some clang warnings, some are unproductive

Too many false positives, not useful enough for us at the moment
at the cost of not having the whole tree build without warnings
as errors.

Change-Id: I9f9910b7f66ebf3a82d42e7732e413ba27dbbbe7
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7778
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)