Makefile: support building SPL FIT images without device trees

When building a U-Boot FIT image, the device trees specified by the
board are unconditionally built for inclusion in the FIT image. However,
not all device tree providers, such as CONFIG_OF_PRIOR_STAGE, require a
device tree to be built and bundled with the U-Boot binary. They rely on
other mechanisms to provide the device tree to U-Boot. Compilation on
boards with these device tree providers fails, because they do not
specify a device tree.

Change the makefile rules to conditionally build the device trees if
CONFIG_OF_SEPARATE, CONFIG_OF_EMBED, or CONFIG_OF_HOSTFILE is selected
as device tree provider.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

Makefile

@@ -1257,7 +1257,9 @@ MKIMAGEFLAGS_u-boot.pbl = -n $(srctree)/$(CONFIG_SYS_FSL_PBL_RCW:"%"=%) \
 		-R $(srctree)/$(CONFIG_SYS_FSL_PBL_PBI:"%"=%) -T pblimage
 
 u-boot-dtb.img u-boot.img u-boot.kwb u-boot.pbl u-boot-ivt.img: \
-		$(if $(CONFIG_SPL_LOAD_FIT),u-boot-nodtb.bin dts/dt.dtb,u-boot.bin) FORCE
+		$(if $(CONFIG_SPL_LOAD_FIT),u-boot-nodtb.bin \
+			$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_OF_HOSTFILE),dts/dt.dtb) \
+		,u-boot.bin) FORCE
 	$(call if_changed,mkimage)
 	$(BOARD_SIZE_CHECK)
 
@@ -1267,7 +1269,9 @@ else
 MKIMAGEFLAGS_u-boot.itb = -E
 endif
 
-u-boot.itb: u-boot-nodtb.bin dts/dt.dtb $(U_BOOT_ITS) FORCE
+u-boot.itb: u-boot-nodtb.bin \
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_OF_HOSTFILE),dts/dt.dtb) \
+		$(U_BOOT_ITS) FORCE
 	$(call if_changed,mkfitimage)
 	$(BOARD_SIZE_CHECK)