Common debconf variables for serial console?

[Petter Reinholdtsen <pere@hungry.com>]

At the moment, both kbd-chooser, lilo-installer and grub-installer
need to know if the installation is using a serial console.  In
addition, there is code to detect this in rootskel
(/lib/debian-installer.d/S30term-linux) and set TERM_TYPE=serial in
this case.  To avoid duplicate code, I suggest using the same debconf
template to store the required information, and that we add these
three templates to rootskel:

  Template: debian-installer/console/serial
  Type: boolean
  Default: false
  Default[s390]: true
  Description: Is the install using serial line console?
    This is used in addition to environment variable TERM_TYPE=serial.

  Template: debian-installer/console/serial/port
  Type: string
  Default: ttyS0
  Description: Device name (in /dev/) of serial console device

  Template: debian-installer/console/serial/speed
  Type: string
  Default: 9600
  Description: Speed of serial console?

There is code in lilo-installer.postinst to detect the current values
from /proc/cmdline.  I suggest this code is moved to rootskel as well.

Any comments?  Is these three variables enough?

I want to make common code out of this, instead of copying the code
from lilo-installer into grub-installer.  If the variables are
available, it will be easier to add serial console support in grub.