OvmfPkg: make the 4MB flash size the default

OvmfPkg: make the 4MB flash size the default

The previously default 2MB can be explicitly selected with

  -D FD_SIZE_2MB

or

  -D FD_SIZE_IN_KB=2048

Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
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>