OvmfPkg: give more telling names to some FDF include files

OvmfPkg: give more telling names to some FDF include files

Leif suggested that FDF include files should preferably refer with their
names to the FDF file sections from which they are included.

Therefore

- rename "OvmfPkg.fdf.inc" to "OvmfPkgDefines.fdf.inc" (included from the
  [Defines] section),

- rename "DecomprScratchEnd.fdf.inc" to "FvmainCompactScratchEnd.fdf.inc"
  (included under the [FV.FVMAIN_COMPACT] section).

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: http://mid.mail-archive.com/20200312142006.GG23627@bivouac.eciton.net
Ref: https://edk2.groups.io/g/devel/message/55812
Suggested-by: Leif Lindholm <leif@nuviainc.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200312223555.29267-3-lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>