sunxi: allow boards to de-select SYS_WHITE_ON_BLACK font scheme

In the sunxi-common.h config header we unconditionally define
CONFIG_SYS_WHITE_ON_BLACK, although it's actually a Kconfig option which
could be individually selected by a user.
Remove this #define from the header and let it default to "y" on sunxi
boards (like we do for other platforms).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>