Adds empty sysresccd_custom.cfg and /boot/grub/custom.cfg files that
are included from the regular boot menus (syslinux and grub). These
are intended for the user to customize.
Having dedicated files for this allows completely replacing the files.
This is easier than requiring careful patching with potential conflicts
with different versions of SystemRescue.
Before this patch a serial console was only enabled in syslinux for
traditional BIOS, not in grub / UEFI. In traditional BIOS the baud rate
was 38400 before.
Today 38400 is rarely used anymore and most modern devices using a
serial console default to 115200, which is the highest baudrate possible
with the original UART-ICs. Modern UART-ICs usually support higher
baudrates too, but they need a special driver for that. So 115200
is often used because of the wider compatibility.
In grub and syslinux a serial console can fully work in parallel to the
regular console. So enabling it doesn't change anything when nothing is
connected to the serial console or no serial port is installed at all.
This commit only touches the bootloaders, not the boot entries.
Consider you don't have a keyboard connected and can only connect over the network. Or the
keyboard you use doesn't work in the bootloader due to BIOS issues with USB. Then waiting
the whole 90 seconds is cumbersome.
30 seconds should still be enough to see the bootloader over a slow network KVM, get the
correct monitor or keyboard attached and similar things. So it should still be enough time
to modify the default boot if desired.
