selftests: gpio: Fix OUTPUT directory in Makefile

When simply running `make' from the selftests top dir, this
error shows up:

cc -O2 -g -std=gnu99 -Wall -I../../../../usr/include/ -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/uuid gpio-mockup-chardev.c ../../../gpio/gpio-utils.o -lmount -o gpio-mockup-chardev
cc: error: ../../../gpio/gpio-utils.o: No such file or directory
<builtin>: recipe for target 'gpio-mockup-chardev' failed
make[1]: *** [gpio-mockup-chardev] Error 1

because the output directory is set to "selftests/gpio" and
all binaries built from ../../../gpio/ end up there. In fact,
they appear as, exempli gratia:
* gpiogpio-event-mon
* gpiogpio-hammer
* gpioinclude/
* gpiolsgpio
which is wrong, as it's missing a directory separator
somewhere.

This patch sets straight the output directory when building
../../../gpio/ so that binaries don't cross paths.

Signed-off-by: Daniel Díaz <[e-mail]>
Signed-off-by: Shuah Khan (Samsung OSG) <[e-mail]>

tools/testing/selftests/gpio/Makefile

···
 TEST_FILES := gpio-mockup-sysfs.sh
 TEST_PROGS_EXTENDED := gpio-mockup-chardev
 
-GPIODIR := ../../../gpio
+GPIODIR := $(realpath ../../../gpio)
 GPIOOBJ := gpio-utils.o
 
 include ../lib.mk
···
 
 override define CLEAN
 	$(RM) $(TEST_PROGS_EXTENDED)
-	$(MAKE) -C $(GPIODIR) clean
+	$(MAKE) -C $(GPIODIR) OUTPUT=$(GPIODIR)/ clean
 endef
 
 $(TEST_PROGS_EXTENDED):| khdr
 $(TEST_PROGS_EXTENDED): $(GPIODIR)/$(GPIOOBJ)
 
 $(GPIODIR)/$(GPIOOBJ):
-	$(MAKE) -C $(GPIODIR)
+	$(MAKE) OUTPUT=$(GPIODIR)/ -C $(GPIODIR)