configs: gxbb: Introduce a common config header file

Introduce a meson-gxbb-common.h header file and derive the
configuration for Hardkernel Odroid-C2 board from that.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Acked-by: Beniamino Galvani <b.galvani@gmail.com>