OvmfPkg: build OVMF_VARS.fd, OVMF_CODE.fd, OVMF.fd

OvmfPkg: build OVMF_VARS.fd, OVMF_CODE.fd, OVMF.fd

OVMF_VARS.fd and OVMF_CODE.fd split the variable store and the firmware
code in separate files.

The PCDs' values continue to depend only on FD_SIZE_1MB vs. FD_SIZE_2MB.
With the split files, it must be ensured on the QEMU command line that
OVMF_VARS.fd and OVMF_CODE.fd be contiguously mapped so that they end
exactly at 4GB. See QEMU commit 637a5acb (first released in v2.0.0).

In this patch we must take care to assign each PCD only once.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15670 6f19259b-4bc3-4df7-8a09-765794883524