EmulatorPkg: Convert to always build FatPkg

EmulatorPkg: Convert to always build FatPkg

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

Build and boot tested with GCC 5.3 on X64, but I did not test
attaching a disk image with a FAT filesystem.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Reviewed-by: Andrew Fish <afish@apple.com>