Kbuild: Use dtc's -d (dependency) option

This hooks dtc into Kbuild's dependency system.

Thus, for example, "make dtbs" will rebuild tegra-harmony.dtb if only
tegra20.dtsi has changed yet tegra-harmony.dts has not. The previous
lack of this feature recently caused me to have very confusing "git
bisect" results.

For ARM, it's obvious what to add to $(targets). I'm not familiar enough
with other architectures to know what to add there. Powerpc appears to
already add various .dtb files into $(targets), but the other archs may
need something added to $(targets) to work.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Mark Salter <msalter@redhat.com>

arch/c6x/boot/Makefile

@@ -13,7 +13,7 @@ obj-y += linked_dtb.o
 endif
 
 $(obj)/%.dtb: $(src)/dts/%.dts FORCE
-	$(call cmd,dtc)
+	$(call if_changed_dep,dtc)
 
 quiet_cmd_cp = CP      $< $@$2
 	cmd_cp = cat $< >$@$2 || (rm -f $@ && echo false)