roms: Rename the EFIROM variable to avoid clashing with iPXE

The iPXE's 'veryclean' recipe removes $(EFIROM) even if the EFIROM
macro originates from elsewhere:

  $ git checkout f590a812~
  $ make -C roms clean EFIROM=$(type -P EfiRom)
  make: Entering directory '/source/qemu/roms'
  [...]
  make -C ipxe/src veryclean
  make[1]: Entering directory '/source/qemu/roms/ipxe/src'
  rm -f bin{,-*}/*.* bin{,-*}/.certificate.* bin{,-*}/.certificates.* bin{,-*}/.private_key.* bin{,-*}/errors bin{,-*}/NIC ./util/zbin ./util/elf2efi32 ./util/elf2efi64 /usr/bin/EfiRom ./util/efifatbin ./util/iccfix ./util/einfo TAGS bin{,-*}/symtab
  rm: cannot remove '/usr/bin/EfiRom': Permission denied
  make[1]: *** [Makefile.housekeeping:1564: clean] Error 1
  make[1]: Leaving directory '/source/qemu/roms/ipxe/src'
  make: *** [Makefile:152: clean] Error 2
  make: Leaving directory '/source/qemu/roms'

Before f590a812 this variable could be overridden or unset,
and the 'veryclean' Makefile rule would not complain.

Commit f590a812 enforces this variable to the Intel EfiRom
tool provided by the EDK2 project.

To avoid the name clash and make the difference between the
projects obvious, rename the variable used by the EDK2 project
as EDK2_EFIROM.

Fixes: f590a812
Reported-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190409134536.15548-2-philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>