roms/Makefile.edk2: don't pull in submodules when building from tarball

roms/Makefile.edk2: don't pull in submodules when building from tarball

Currently the `make efi` target pulls submodules nested under the
roms/edk2 submodule as dependencies. However, when we attempt to build
from a tarball this fails since we are no longer in a git tree.

A preceding patch will pre-populate these submodules in the tarball,
so assume this build dependency is only needed when building from a
git tree.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Bruce Rogers <brogers@suse.com>
Cc: qemu-stable@nongnu.org # v4.1.0
Reported-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-Id: <20190912231202.12327-3-mdroth@linux.vnet.ibm.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>