Makefile: add kselftest-merge

Add kselftest-merge to enable the dependencies of kernel config
for all the kselftest testcases. User should call it after create
the kernel configuration(.config), eg:

$ make kselftest-merge

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