board/common: add missing newline at end of /etc/motd

With the changes in 9847a8f we can keep the /etc/motd file with a proper
newline at EOF so that also Classic builds make sense.

Signed-off-by: Joachim Wiberg <[email protected]>

Author: troglobit

board/common/rootfs/etc/motd

@@ -1,4 +1,4 @@
 .-------.
 |  . .  | Infix -- a Network Operating System
 |-. v .-| https://kernelkit.github.io
-'-'---'-'
+'-'---'-'