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.