selftests: enable O and KBUILD_OUTPUT

Enable O and KBUILD_OUTPUT for kselftest. User could compile kselftest
to another directory by passing O or KBUILD_OUTPUT. And O is high
priority than KBUILD_OUTPUT.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/powerpc/mm/Makefile

@@ -8,8 +8,8 @@ include ../../lib.mk
 
 $(TEST_GEN_PROGS): ../harness.c
 
-prot_sao: ../utils.c
+$(OUTPUT)/prot_sao: ../utils.c
 
-tempfile:
-	dd if=/dev/zero of=tempfile bs=64k count=1
+$(OUTPUT)/tempfile:
+	dd if=/dev/zero of=$@ bs=64k count=1