Makefile: Correct SPL/TPL/DTB build race condition

When building on a multi-core machine for an SPL-enabled board that
also uses CONFIG_OF_EMBED, the following error can be encountered
due to a race condition:

  make[3]: *** No rule to make target 'spl/dts/dt.dtb.o', needed by
    'spl/dts/built-in.o'.  Stop.
  ../scripts/Makefile.spl:364: recipe for target 'spl/dts' failed
  make[2]: *** [spl/dts] Error 2
  make[2]: *** Waiting for unfinished jobs....

A reliable way to trigger this race condition is to add "sleep 60" to
the end of the "arch-dtbs" rule's recipe in "dts/Makefile" and to build
U-Boot against a board which uses the CONFIG_OF_EMBED and CONFIG_SPL
options using "make -j8" or a similar command.

This commit corrects this race condition via the use of CONFIG_OF_EMBED
in the same way that commit 3c00a2c8b5e2 ("Makefile: Correct dependency
race condition with TPL") and commit 054b3a1e80fc ("dm: Makefile: Build
of-platdata before SPL") use CONFIG_OF_SEPARATE.

Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1389,8 +1389,8 @@ u-boot.lds: $(LDSCRIPT) prepare FORCE
 spl/u-boot-spl.bin: spl/u-boot-spl
 	@:
 spl/u-boot-spl: tools prepare \
-		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb) \
-		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_TPL_OF_PLATDATA),dts/dt.dtb)
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb) \
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_TPL_OF_PLATDATA),dts/dt.dtb)
 	$(Q)$(MAKE) obj=spl -f $(srctree)/scripts/Makefile.spl all
 
 spl/sunxi-spl.bin: spl/u-boot-spl
@@ -1406,7 +1406,7 @@ spl/boot.bin: spl/u-boot-spl
 	@:
 
 tpl/u-boot-tpl.bin: tools prepare \
-		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb)
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb)
 	$(Q)$(MAKE) obj=tpl -f $(srctree)/scripts/Makefile.spl all
 
 TAG_SUBDIRS := $(patsubst %,$(srctree)/%,$(u-boot-dirs) include)