Possibility to work with git submodules

This option will make the generated commit IDs deterministic, greatly
reducing the number of repository objects created when calling `make
update` repeatedly.

This will allow us to use its content from other languages than Lua as
well.

Using a loop makes the script a bit nicer, and it works around a git bug
corrupting patch dates when multiple patches are applied at once.

This fixes `make update` not working correctly when GLUON_SITEDIR is set
manually.

check_site.sh and site.sh were setting GLUON_SITE_CONFIG on their own
even though the environment variable was already exported.

If a given commit is not included in the master branch, the update script fails.

Let the site configuration value script return 1 when
the requested value is not available.This enables us
to react on failures, so we can for example use a default
value.

With this patch the script will return with exitcode 1 if
the value is not found and no output on stdout. Otherwise
exitcode is 0 and the value is returned on stdout.