selftests: Add kselftest install tool

kselftest_install.sh tool installs selftests in default location
which is tools/testing/selftests/kselftest or an user specified
location. This tool invokes back-end selftests install target with
the install location.

Usage:
cd tools/testing/selftests
./kselftest_install.sh [ install_dir ]

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>