rockchip: configs: only add available BOOT_TARGET_DEVICES

BOOT_TARGET_DEVICES should only be added if the corresponding u-boot
command is enabled.

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>