gluon-core: sleep a few seconds during upgrade

wait for device initialisations
workaround for a timing issue during first boot on ath79-generic
after sysupgrade from ar71xx-generic image

GitHub Issue: #2779

(cherry picked from commit d97673f)

package/gluon-core/files/etc/uci-defaults/zzz-gluon-upgrade

@@ -1,5 +1,10 @@
 #!/bin/sh
 
+# Work around an issue with wifi setup timing by waiting a bit
+# while device initialisation is ongoing.
+# https://github.com/freifunk-gluon/gluon/issues/2779
+sleep 3
+
 gluon-reconfigure
 
 exit 0