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>

SVN-Revision: 47623

package/base-files/files/etc/profile

 #!/bin/sh
 [ -f /etc/banner ] && cat /etc/banner
 [ -e /tmp/.failsafe ] && cat /etc/banner.failsafe
+fgrep -sq '/ overlay ro,' /proc/mounts && {
+	echo 'Your JFFS2-partition seems full and overlayfs is mounted read-only.'
+	echo 'Please try to remove files from /overlay/upper/... and reboot!'
+}
 
 export PATH=/usr/sbin:/usr/bin:/sbin:/bin
 export HOME=$(grep -e "^${USER:-root}:" /etc/passwd | cut -d ":" -f 6)