selftests: gpio: restructure Makefile

This patch cleans up the Makefile by restructuring a couple of
things, namely:
1) change explicit paths in targets for variables
2) substitute a variable (BINARIES) for another, part of the
   selftests build system (TEST_PROGS_EXTENDED)
3) proper cleaning up of the EXTRA objects

The resulting Makefile is much more readable and manageable.
Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/gpio/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
+CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/
+LDLIBS += -lmount -I/usr/include/libmount
+
 TEST_PROGS := gpio-mockup.sh
-TEST_FILES := gpio-mockup-sysfs.sh $(BINARIES)
-BINARIES := gpio-mockup-chardev
-EXTRA_PROGS := ../gpiogpio-event-mon ../gpiogpio-hammer ../gpiolsgpio
-EXTRA_DIRS := ../gpioinclude/
-EXTRA_OBJS := ../gpiogpio-event-mon-in.o ../gpiogpio-event-mon.o
-EXTRA_OBJS += ../gpiogpio-hammer-in.o ../gpiogpio-utils.o ../gpiolsgpio-in.o
-EXTRA_OBJS += ../gpiolsgpio.o
+TEST_FILES := gpio-mockup-sysfs.sh
+TEST_PROGS_EXTENDED := gpio-mockup-chardev
+
+GPIODIR := ../../../gpio
+GPIOOBJ := gpio-utils.o
 
 include ../lib.mk
 
-all: $(BINARIES)
+all: $(TEST_PROGS_EXTENDED)
 
 override define CLEAN
-	$(RM) $(BINARIES) $(EXTRA_PROGS) $(EXTRA_OBJS)
-	$(RM) -r $(EXTRA_DIRS)
+	$(RM) $(TEST_PROGS_EXTENDED)
+	$(MAKE) -C $(GPIODIR) clean
 endef
 
-CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/
-LDLIBS += -lmount -I/usr/include/libmount
-
-$(BINARIES):| khdr
-$(BINARIES): ../../../gpio/gpio-utils.o
+$(TEST_PROGS_EXTENDED):| khdr
+$(TEST_PROGS_EXTENDED): $(GPIODIR)/$(GPIOOBJ)
 
-../../../gpio/gpio-utils.o:
-	make ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) -C ../../../gpio
+$(GPIODIR)/$(GPIOOBJ):
+	$(MAKE) -C $(GPIODIR)