configs: change configurations based on the host architecture

Change the build configurations of RD-N2 variants based on the host
machine architecture. Environment variable BUILD_MACHINE_ARCH is used
for host specific changes in the configuration.

ToDo: The build of StandaloneMM is yet to be tested for AArch64 builds.

Change-Id: Ia0ae178b7e5bcaf196f88245d44426f8effb8bb8
Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>