ArmVirtPkg: allow setting Firmware Version from build command line

ArmVirtPkg: allow setting Firmware Version from build command line

ArmVirtXen.dsc initializes
gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString with with the
value of the variable "FIRMWARE_VER".

Move that functionality to ArmVirt.dsc.inc to make it available to all
ArmVirt packages, and make it conditional: only set the PCD string if
FIRMWARE_VER is actually defined.

This allows specifying the firmware version string on the build command
line with -D FIRMARE_VER=...

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>