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>

Makefile

@@ -752,6 +752,9 @@ ALL-$(CONFIG_SPL) += spl/u-boot-spl.bin
 ALL-$(CONFIG_SPL_FRAMEWORK) += u-boot.img
 ALL-$(CONFIG_TPL) += tpl/u-boot-tpl.bin
 ALL-$(CONFIG_OF_SEPARATE) += u-boot.dtb u-boot-dtb.bin
+ifeq ($(CONFIG_SPL_FRAMEWORK),y)
+ALL-$(CONFIG_OF_SEPARATE) += u-boot-dtb.img
+endif
 ALL-$(CONFIG_OF_HOSTFILE) += u-boot.dtb
 ifneq ($(CONFIG_SPL_TARGET),)
 ALL-$(CONFIG_SPL) += $(CONFIG_SPL_TARGET:"%"=%)
@@ -854,6 +857,11 @@ MKIMAGEFLAGS_u-boot.pbl = -n $(srctree)/$(CONFIG_SYS_FSL_PBL_RCW:"%"=%) \
 u-boot.img u-boot.kwb u-boot.pbl: u-boot.bin FORCE
 	$(call if_changed,mkimage)
 
+MKIMAGEFLAGS_u-boot-dtb.img = $(MKIMAGEFLAGS_u-boot.img)
+
+u-boot-dtb.img: u-boot-dtb.bin FORCE
+	$(call if_changed,mkimage)
+
 u-boot.sha1:	u-boot.bin
 		tools/ubsha1 u-boot.bin