build-sys: fix building with make CFLAGS=.. argument

When calling make with a CFLAGS=.. argument, the -g/-O filter is not
applied, which may result with build failure with ASAN for example. It
could be solved with an 'override' directive on CFLAGS, but that would
actually prevent setting different CFLAGS manually.

Instead, filter the CFLAGS argument from the top-level Makefile (so
you could still call make with a different CFLAGS argument on a
rom/Makefile manually)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20160805082421.21994-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>