kbuild: Remove reference to uninitialised variable

Verbose output variable is unnecessary because the command's echo is
already surpressed. Additionally because the block defines skip-makefile
the variable Q is not defined within the makefile, which can cause
problems if Q is defined in the users environment.
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -124,7 +124,7 @@ $(if $(KBUILD_OUTPUT),, \
 PHONY += $(MAKECMDGOALS) sub-make
 
 $(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
-	$(Q)@:
+	@:
 
 sub-make: FORCE
 	$(if $(KBUILD_VERBOSE:1=),@)$(MAKE) -C $(KBUILD_OUTPUT) \