greybus: tools: don't always build them.

This breaks the kernel-only build as it can't find any userspace headers
with the cross-compiler, so don't build the tools by "default" unless
you ask for them.
Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/Makefile

@@ -85,7 +85,7 @@ ccflags-y := -Wall
 # needed for trace events
 ccflags-y += -I$(src)
 
-all: module tools
+all: module
 
 tools::
 	$(MAKE) -C tools KERNELDIR=$(realpath $(KERNELDIR))