ttySI0 was a name of HiFive Unleashed serial console. It was changed to
ttySIF0.

The case at hand was the need to pass serial console parameters,
see the next commit.

ttyAMA0 needed for qemu on aarch64 and armh.
ttySI0 needed for HiFive Unleashed (riscv64).

...the one starting agetty in installer if told so (0.4+);
also improve runlevel 2 with potential serial access.

...as vm images don't need extra processing.

A one-liner for syslinux and a simple installer feature
change a lot in this regard!

This one only cares for serial console(s) in sysvinit case now
but might end up controlling the number of gettys for ve/vm, etc.