OvmfPkg: allow setting Firmware Version from build command line

OvmfPkg: allow setting Firmware Version from build command line

Initialize gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString with
with the value of the variable "FIRMWARE_VER", if is is defined. Applies
to all flavors of OvmfPkg.

This behavior is already implemented in ArmVirtXen.dsc. It allows
specifying the firmware version string on the build command line with
-D FIRMARE_VER=...

Introduce a common include file to be used in the .dsc files for the
different OVMF flavors, and add the changes there.  (ArmVirtPkg already
has such a file).

Signed-off-by: Oliver Steffen <osteffen@redhat.com>