selftests: Add tool to generate kselftest tar archive

gen_kselftest_tar.sh tool generates kselftest tar archive. This tool
supports uncompressed tar, gz, bz, and xz compression formats and the
default compression format is gzip. This tool runs kselftest install
tool as its back-end.

Usage:
cd tools/testing/selftests
./gen_kselftest_tar [ tar | targz | tarbz2 | tarxz ]

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