Revert "OvmfPkg: make the 4MB flash size the default"
    
    This reverts commit bba8dfbec3bbc4fba7fa6398ba3cf76593e0725e.
    
    The 264KB size introduced for the NV spare area in commit b24fca05751f
    ("OvmfPkg: introduce 4MB flash image (mainly) for Windows HCK",
    2017-04-29) breaks the "-bios" (emulated varstore) use case. Until we sort
    that out, revert the default build to the 2MB image.
    
    Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>