Update MOTD on FreeBSD 13

Summary:
FreeBSD 13 moved from /etc/motd to /etc/motd.template and provides
a service to prepend kernel info to the MOTD.

The execution module allows to write a simpler state.

Test Plan: Tested on WindRiver

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2474