fix flag handling

Make sure we respect CPPFLAGS and don't hardcode

BUG=None
TEST=`emerge-x86-alex rootdev` uses right flags

Change-Id: I668af9e02b842ddc54f60e07d313a564cc927413
Reviewed-on: https://chromium-review.googlesource.com/175522
Reviewed-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>