sunxi: introduce extra config option for boot0 header

The ENABLE_ARM_SOC_BOOT0_HOOK option is a generic option shared with
other boards. To allow alternative code to be inserted, we create
another, now function specific config symbol on top of it to simplify
later additions. No functional change at this time.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>