OvmfPkg: Convert to using FatPkg in the EDK II tree

OvmfPkg: Convert to using FatPkg in the EDK II tree

Now that FatPkg is open source (and therefore can be included in the
EDK II tree) we build and use it directly.

Build tested with GCC 5.3 on IA32 and X64. Boot tested to UEFI Shell
on IA32 and UEFI Linux on X64.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>