sunxi: make SoC variant choice mandatory

The user should always select an SoC variant to support. Not choosing
one doesn't make sense for a bootloader.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>