ar71xx: mikrotik: prepend "RouterBOARD" to machine name for NOR devices

When the device name doesn't already contain "RouterBOARD", this patch adds
this string to the machine name.

Most NOR devices already have "RouterBOARD" in their hardware-stored device name,
but not all of them.

This patch also makes the code more robust against buffer overflows.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>