build: support a new image u-boot-dtb.img

In SPL framework, SPL uses u-boot.img to load u-boot.bin.
Here,
    u-boot.img = uImage header + u-boot.bin

To use OF control with a separate devicetree,
u-boot.dtb must be placed right after u-boot.bin.
In this case, u-boot-dtb.bin is generally used.
Here,
    u-boot-dtb.bin = u-boot.bin + u-boot.dtb

We need u-boot-dtb.img to use both SPL framework
and separate OF control at the same time.
    u-boot-dtb.img = uImage header + u-boot-dtb.bin

For example, Zynq boards already define all of
  - CONFIG_SPL
  - CONFIG_OF_CONTROL
  - CONFIG_OF_SEPARATE

So, the support of u-boot-dtb.img is urgent.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>