archiso does not expose serial console

When installing on a PC Engines machine, I realized, that we do not setup serial console by default for archiso. IMHO this should still be provided, so that true headless systems (only accessible via serial port) can be installed/ accessed as well.

A common way would be to add console=ttyS0,115200 to the kernel cmdline options. I'm not aware whether we can do this for the bootloader itself as well, but will investigate.

btw: 115200 is the baud rate commonly used in old serial modems.