OvmfPkg: increase MEMFD size to 8MB

OvmfPkg: increase MEMFD size to 8MB

With reference to
<http://sourceforge.net/mailarchive/message.php?msg_id=30359322>:

"MEMFD is built so MAINFV's contents will be relocated during the build to
address 0x800000", and it "is a firmware volume with most OVMF code/data
uncompressed. [...] Increasing its size has a little impact on the size of
the resulting firmware image since the blank part of the firmware volume
will compress well."

Let's increase the size to 8MB, since the current limit can get in the way
(for example when building-in the Intel3.5 drivers for e1000 with
-D FD_SIZE_2MB -D NETWORK_ENABLE -D SECURE_BOOT_ENABLE).

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://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14133 6f19259b-4bc3-4df7-8a09-765794883524