mxgrub: Default to ttyS0 for serial console instead of ttyS1

We started to use the serial console for debugging for the Dell
PowerEdge R910 and R930, which assign `ttyS1` to the serial console on
the back, as they also have, I think, a serial port on the front.

Unfortunately, Linux does not allow to change the serial console port at
run-time, so the set value on the Linux kernel command line matters.

Most systems, including desktops, often only have one port, or the port,
we are interested in, so it’s `ttyS0` in Linux.

Therefore, change the default to `ttyS0` to cover more systems.

mxgrub/mxgrub

@@ -240,7 +240,7 @@ sub scan_mariux {
 	}
 }
 
-our $KERNEL_PARAMETER="ro crashkernel=256M console=ttyS1,115200n8 console=tty0 init=/bin/systemd audit=0 random.trust_cpu=on";
+our $KERNEL_PARAMETER="ro crashkernel=256M console=ttyS0,115200n8 console=tty0 init=/bin/systemd audit=0 random.trust_cpu=on";
 
 sub update_grub_cfg {
 	my $kernellist='';