buildscript: Added kernel selftest support

This change supports mte, bti and pauth tests. All the tests
can be run at once using run_kselftest.sh

README.md has been updated with usage informaiton

Signed-off-by: Rupinderjit Singh <rupinderjit.singh@arm.com>
Change-Id: Id1f6bdaeaa25d9d39e5cfcf74df59159e71149d1