scripts: set default CFLAGS to optimise.

The extra CFLAGS override the product defaults, causing the product to
be built without optimisation or debug. Be explicit and add these in.

Reported-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Michael Hope <michael.hope@linaro.org>
Message-Id: <CANLjY-=3Gbio6nzUPhhevDHV7cUN=6Vigooe9nSf-RnGCqnjog@mail.gmail.com>
Patchwork-Id: 198808

reporter_name="Thomas JOURDAN"

reporter_url=""

reporter_comment=""