base-files: show hint when JFFS2-partition is full and overlayfs mounted read-only

base-files: show hint when JFFS2-partition is full and overlayfs mounted read-only

since r45833 and r45848 the overlayfs is automatically mounted read-only
(and blocks remounts as r/w) when JFFS2 is full. see ticket #19564

because of this normal file deletion is not possible anymore.
if a user logins interactively (e.g. SSH) show a hint for this,
that files must be removed in /overlay/upper/...

v2: fix subject line

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47623 3c298f89-4303-0410-b956-a3cf2f4a3e73