infra/rdn2cfg1: 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-Cfg1 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: I3cec577cb9aad11694cb995d2f8ff3e9893760a9