Add kvm-unit-tests

This change introduces the build script for kvm-unit-tests as well as
updates to configuration file and buildroot script to enable this tests
to be used when the stack is booted on the FVP.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Idd7b47516fe2532f16d8a6b3bcec12f198585b57