build: allow specifying EXTRA_BUILD_FLAGS for development

support easier handling of flags for developers as EXTRA_BUILD_FLAGS is
available for all targets (all, install, all-dev, install-dev, clean).

Change-Id: I12d59fad2ca46a67e0c3ec0baa6189a4fb7035bf
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>