Return exit code on missing value

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.

scripts/site.sh

@@ -2,4 +2,5 @@
 
 export GLUONDIR="$(dirname "$0")/.."
 
-echo "@$1@" | $GLUONDIR/scripts/configure.pl $GLUONDIR/scripts/generate.pl
+RESULT=$(echo "@$1@" | $GLUONDIR/scripts/configure.pl $GLUONDIR/scripts/generate.pl)
+test ! "$RESULT" = "@$1@" && echo $RESULT