mxgrub: Tell Linux to log to serial console (#10)

To debug Linux kernel double crashes for the MXQ servers, they are now
connected over serial console.

As the Dell firmware, and GRUB use BIOS calls to print messages, these
are already transmitted over serial console thanks to some Dell BIOS
magic.

But, luckily, the Linux kernel doesn’t do that. So Linux has to be told
to write messages to the serial console.

Omitting the `console=tty0` would cause no Linux messages to be printed
to the VGA connection, which we use for the KVMs, so add that too.