selftests/gpio: Fix build when source tree is read only

Currently the gpio selftests fail to build if the source tree is read
only:

make -j 160 -C tools/testing/selftests TARGETS=gpio
make[1]: Entering directory '/linux/tools/testing/selftests/gpio'
make OUTPUT=/linux/tools/gpio/ -C /linux/tools/gpio
make[2]: Entering directory '/linux/tools/gpio'
mkdir -p /linux/tools/gpio/include/linux 2>&1 || true
ln -sf /linux/tools/gpio/../../include/uapi/linux/gpio.h /linux/tools/gpio/include/linux/gpio.h
ln: failed to create symbolic link '/linux/tools/gpio/include/linux/gpio.h': Read-only file system

This happens because we ask make to build ../../../gpio (tools/gpio)
without pointing OUTPUT away from the source directory.

To fix it we create a subdirectory of the existing OUTPUT directory,
called tools-gpio, and tell tools/gpio to build in there.

Signed-off-by: Michael Ellerman <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/gpio/Makefile

···
 include ../lib.mk
 
 GPIODIR := $(realpath ../../../gpio)
-GPIOOBJ := gpio-utils.o
+GPIOOUT := $(OUTPUT)/tools-gpio/
+GPIOOBJ := $(GPIOOUT)/gpio-utils.o
 
 override define CLEAN
 	$(RM) $(TEST_GEN_PROGS_EXTENDED)
-	$(MAKE) -C $(GPIODIR) OUTPUT=$(GPIODIR)/ clean
+	$(RM) -rf $(GPIOOUT)
 endef
 
-$(TEST_GEN_PROGS_EXTENDED): $(GPIODIR)/$(GPIOOBJ)
+$(TEST_GEN_PROGS_EXTENDED): $(GPIOOBJ)
 
-$(GPIODIR)/$(GPIOOBJ):
-	$(MAKE) OUTPUT=$(GPIODIR)/ -C $(GPIODIR)
+$(GPIOOUT):
+	mkdir -p $@
+
+$(GPIOOBJ): $(GPIOOUT)
+	$(MAKE) OUTPUT=$(GPIOOUT) -C $(GPIODIR)