OvmfPkg: Increase DEBUG build size to 2MB by default

OvmfPkg: Increase DEBUG build size to 2MB by default

The 1MB image with full debug and the shell included is too
large to implement flash based non-volatile variable.

After this change, building with -D FD_SIZE_1MB will
force the smaller flash size.

The default size for RELEASE build remains at 1MB, so using
-b RELEASE on the build command line will result in a
1MB flash size. For RELEASE builds -D FD_SIZE_2MB can be
used to produce a 2MB flash image.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14833 6f19259b-4bc3-4df7-8a09-765794883524