Provide a motd message when users get a shell into a exporter

[mangelajo]

The motd should contain any relevant documentation the administrator seems valuable for the users, as well as any extra details the hooks want to put into motd (like previous reports for the exporters, etc...)

jmp shell -l board=ti-123

You have been assigned to exporter ti-123-001:

You can use images from

upstream: https://.....
downstream: https://....

You can flash the device with:

j storage flash ......

and access the serial console with

j serial start-console

$

This is related to #65 and could take some of the output of the hooks to add it to the motd message.

Also reports from #52 could be listed in the motd