[syslinux] How to show boot prompt and edit kernel cmdline from serial?

[piranna at gmail.com]

For testing purposses, I'm trying to hook on QEmu an ISO image that
start automatically with syslinux (just a DEFAULT label and no TIMEOUT
option) and change the kernel arguments to redirect the console to a
serial port instead of the screen.

So far I've seen that the SERIAL statement give me a welcome message
and the loading of the kernel and initramfs on the serial port (but
curiously, not the "Loading NodeOS" message that I put, just directly
the kernel string...), but the system start automatically. I've read
in the docs that the boot prompt can be forced to be shown if the
caps_lock or the scroll_lock keys are set, but QEmu doen't offer a way
to have them enabled at boot time and by using the monitor seems to be
too complicated (and since syslinux is booting automatically, too much
time constrained too). So I ask: is there a way to force to show the
boot prompt on the serial port on demand? Maybe a magic characters
sequence I send over the serial port itself after I receive the
syslinux welcome message?


-- 
"Si quieres viajar alrededor del mundo y ser invitado a hablar en un
monton de sitios diferentes, simplemente escribe un sistema operativo
Unix."
– Linus Tordvals, creador del sistema operativo Linux