selftests: add EXTRA_CLEAN for clean target

Some testcases need the clean extra data after running. This patch
introduce the "EXTRA_CLEAN" variable to address this requirement.

After KBUILD_OUTPUT is enabled in later patch, it will be easy to
decide to if we need do the cleanup in the KBUILD_OUTPUT path(if the
testcase ran immediately after compiled).

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