selftests: create test-specific kconfig fragments

Create the config file in each directory of testcase which need
more kernel configuration than the default defconfig. User could
use these configs with merge_config.sh script:

Enable config for specific testcase:
(export ARCH=xxx #for cross compiling)
./scripts/kconfig/merge_config.sh .config \
tools/testing/selftests/xxx/config

Enable configs for all testcases:
(export ARCH=xxx #for cross compiling)
./scripts/kconfig/merge_config.sh .config \
tools/testing/selftests/*/config

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