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>