linux: add dtb makefile target to targets list

Add dtb makefile target to targets list to permit correct working of

make target/linux/dtb

Fixes: c47532b1ea7f ("kernel-buildOnmk: add support for compiling only DTS")
Signed-off-by: Christian Marangi <[email protected]>

target/linux/Makefile

@@ -7,5 +7,5 @@ include $(INCLUDE_DIR)/target.mk
 
 export TARGET_BUILD=1
 
-prereq clean download prepare compile install oldconfig menuconfig nconfig xconfig update refresh: FORCE
+prereq clean download prepare compile install oldconfig menuconfig nconfig xconfig update refresh dtb: FORCE
 	@+$(NO_TRACE_MAKE) -C $(firstword $(wildcard feeds/$(BOARD) $(BOARD))) $@