Add the ability to set and delete system MotD (Message Of The Day)
using the cli.
This patch augments the standard ietf-system yang schema with
a new infix-system schema which contains a new motd leaf.
The default value is hard coded in two places, confd and the rootfs
file /etc/motd. The intention is to fix this in upcoming patches.
Introduces:
exec> configure> set system motd STRING
exec> configure> delete system motd
