selftests/capabilities: clean up for Makefile

Clean up the following things:
1.  Avoid the broken when use TARGETS in the command line, eg:

 $ make -C tools/testing/selftests TARGETS=capabilities
 make[1]: *** No rule to make target 'capabilities', needed by 'all'.  Stop.

    Replace TARGETS with BINARIES.

2.  User need to provide cap-ng.h and libcap-ng.so for cross compiling.
    Replace ':=' with '+=' for CFLAGS and introduce LDLIBS to archieve
    it. Delete useless EXTRA_CLAGS at the same time.

3.  Delete the duplicated definition which is already defined by
    lib.mk.
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/capabilities/Makefile

-all:
-
-include ../lib.mk
-
-.PHONY: all clean
-
-TARGETS := validate_cap test_execve
+TEST_FILES := validate_cap
 TEST_PROGS := test_execve
 
-CFLAGS := -O2 -g -std=gnu99 -Wall -lcap-ng
+BINARIES := $(TEST_FILES) $(TEST_PROGS)
 
-all: $(TARGETS)
+CFLAGS += -O2 -g -std=gnu99 -Wall
+LDLIBS += -lcap-ng -lrt -ldl
+
+all: $(BINARIES)
 
 clean:
-	$(RM) $(TARGETS)
+	$(RM) $(BINARIES)
+
+include ../lib.mk
 
-$(TARGETS): %: %.c
-	$(CC) -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl