infra/rdn2: updates for RD-INFRA-2021.05.27 release

For the RD-INFRA-2021.05.27 release, allow buildroot boot and
virtualization to be usable with the RD-N2 platform software stack.
In addition to this, update the expected FVP version to be used
along with software tagged as RD-INFRA-2021.05.27.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I38e5a736fb47da2e8c088903c8c488a7986bbd92