scripts: do not create gluon_base entry in feeds.conf

The gluon-script "feeds.sh" creates by default a "gluon_base" entry in the feeds.conf
of OpenWrt. This links to the gluon-base packages directory and causes an error message
during build, as this directory does not exist in our non-gluon firmware.
Add a test for definiton of a "FOREIGN_BUILD" variable to the feeds.sh script, which
will no add the mentioned line to the feeds.conf, when defined.

scripts/feeds.sh

@@ -11,7 +11,7 @@ rm -rf openwrt/feeds
 rm -rf openwrt/package/feeds
 
 (
-	echo 'src-link gluon_base ../../package'
+	[ -n "${FOREIGN_BUILD}" ] || echo 'src-link gluon_base ../../package'
 	for feed in $FEEDS; do
 		echo "src-link $feed ../../packages/$feed"
 	done