Replace the --enable-git-update and --disable-git-update configure params
with the param --with-git-submodules=(update|validate|ignore) to
allow 3 options for building from a git repo.

This is needed because downstream packagers, e.g. Debian, Ubuntu, etc,
also keep the source code in git, but do not want to enable the
'git_update' mode; with the current code, that's not possible even
if the downstream package specifies --disable-git-update.

The previous parameters are deprecated but still available; the
--enable-git-update parameter maps to --with-git-submodules=update and
--disable-git-update parameter maps to --with-git-submodules=validate.

The configure script behavior is slightly modified, where previously
the dtc, capstone, and slirp submodules were not validated when
--disable-git-update was specified (but were updated with git-update
enabled), now they are validated when using --with-git-submodules=validate
and are only ignored when using --with-git-submodules=ignore.

Signed-off-by: Dan Streetman <ddstreet@canonical.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>