oldsys-preseed: Convert to confmodule instead of using a preseed file to allow overrides

[Loïc Minier <lool@debian.org>]

        Hey!

 I was booting d-i over TFTP on a Thecus N2100 using the d-i netboot
 images, but oldsys-preseed would detect bogus settings from the config
 partition of the original firmware such as IP address, netmask etc.  I
 couldn't override this from the kernel cmdline and Colin helpfully
 pointed out that because oldsys-preseed generated a preseed file, I
 wouldn't be able to override debconf questions coming out of this file
 and suggested that oldsys-preseed be converted to using confmodule.

 This patch series does that; I've tested it by running the
 oldsys-preseed testsuite and by booting the N2100 with a modified
 oldsys-preseed.  With these RedBoot commands:
    load -r -v -b 0x00800000 -m tftp initrd.gz
    load -r -v -b 0x00200000 -m tftp vmlinuz-2.6.32-5-iop32x
    exec -c "console=ttyS0,115200 initrd=0xa0800000,42M"
 it boots as before, wrong network settings, but if I add
 netcfg/disable_dhcp=false to the kernel cmdline to revert the effects
 of oldsys-pressed, it will now DHCP properly and I can happily SSH into
 the DHCP-provided IP address.

 Open questions:
 - I'm not sure what the NONINTERACTIVE variable is for in
   oldsys-preseed, but it might make sense to not set the debconf
   frontend if we're in interactive mode?  I opted to not set the seen
   flag in interactive mode, albeit NONINTERACTIVE is currently
   hardcoded to true
 - the original bug remains that I don't really want to use the bogus
   settings from the original firmware, but either the ones from the
   previous Debian installation on the MD array, or the RedBoot
   settings; I don't have a strong idea of the best design here, and am
   happy with manually preseeding the network settings I care about, but
   if others see a strong need for it, we could implement either config
   sources
 - similarly, it might make sense to add a way to skip oldsys-preseed
   entirely; maybe there's a feature to do that in d-i that I don't know
   about?  otherwise, we could add some new debconf question just for
   this

   Cheers,
-- 
Loïc Minier