uboot: ensure u-boot-dtb.img gets built if enabled

uboot: ensure u-boot-dtb.img gets built if enabled

u-boot-dtb.img only gets built by default (all target) if
CONFIG_OF_SEPARATE _AND_ CONFIG_SPL_FRAMEWORK are enabled.

Certain SoCs (E.G. Action Semi S500) use a proprietary first level
bootloader instead of u-boot SPL, but still use u-boot-dtb.img as the first
level bootloader parses the uImage header for size/loadaddr.

Ensure u-boot-dtb.img gets built if enabled by explicitly listing it as a
u-boot make target (next to all) to support such setups.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>