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>