rockchip: configs: use rockchip-common.h for rk3368

rockchip-common.h already defines values that are missing from
rk3368_common.h

For example BOOT_TARGET_DEVICES was defined empty and therefore
distroboot had no boot targets.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>