kbuild: avoid race between dtbs and dt/dt.dtb targets

If the final targets depend on both "dtbs" and "dts/dt.dtb",
and -j option is given to the command line, multiple threads
descend into the dts/ directory, which causes build error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>

Makefile

@@ -810,7 +810,9 @@ ifeq ($(CONFIG_DM_I2C_COMPAT),y)
 endif
 
 PHONY += dtbs
-dtbs dts/dt.dtb: checkdtc u-boot
+dtbs: dts/dt.dtb
+	@:
+dts/dt.dtb: checkdtc u-boot
 	$(Q)$(MAKE) $(build)=dts dtbs
 
 quiet_cmd_copy = COPY    $@