git-submodule.sh: allow running in validate mode without previous update

The call to git-submodule.sh done in configure may happen without a
previous checkout of the roms/SLOF submodule, or even without a
previous run of the script.

So, handle creating a .git-submodule-status file even in validate
mode.  If git is absent, ensure that all passed directories exists
(because you should be in a fresh untar and will not have stale
arguments to git-submodule.sh) but do no other checks.  If git
is present, ensure that .git-submodule-status contains an entry
for all submodules passed on the command line.

With this change, "ignore" mode is not needed anymore.

Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Fixes: b11f9bd9 ("configure: move SLOF submodule handling to pc-bios/s390-ccw", 2023-06-06)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>