configure apt_preferences to use PPA for moblin build

I've attached an /etc/apt/preferences file. Lool please check that it is correct (I've never done this before) and then reassign to NCommander to fix the live root installer.

Note, we also need to install this file in the image such that updates initiated by the user use this prefs file as well. Possibly it can be added to moblin-ux-settings package.

Thanks Bill; the syntax of the file looks ok. The hardest thing to decide is the score, which is why I didn't add the file straight-away: I'd like the score to be as low as possible for things to work as we expect.

I will try with a score of 995 which I think is the proper score to give PPA versions priority over other versions from the target release (even if higher). I fear a score of 1000 or above also forces installation of uninstalled packages.

In my testing, score 501 is enough; I'll use score 550.

Couple of notes:
- v=9.10 is not convenient to implement in livecd-rootfs as we only have the suite name; it would be possible to use n=karmic though; I don't think this would be updated across a dist-upgrade, so it seems sensible to have this support as generic as possible in livecd-rootfs and pin all series in the PPA equally
- o= is set to the Origin: field which is expected to be LP-PPA-$ppa with all slashes replaced with dashes; there's a catch though: PPAs named "ppa" have LP-PPA-foo as origin, not foo-ppa; this is for backward compatibility

Bah I discovered an APT bug which prevented the preferences file (the APT policy really) to be taken into account; see bug #442082 when it's marked public.

This bug was fixed in the package livecd-rootfs - 0.99

---------------
livecd-rootfs (0.99) karmic; urgency=low

  * Revert 0.98 change; it's actually much more wide-ranging than I thought as
    it would cause each livefs build on a subarch to rotate the timestamped
    dir and hence would mask the previous builds; and even if that would be
    fixed, arch-indepedent filenames such as manifest/kernel/initrd/etc.
    symlinks would clash.
  * Rename EXTRASOURCE to PPA and also define PPAMIRROR near top, set to
    ppa.launchpad.net. Changing to PPA avoids listing non-main components in
    deb lines and having an useful comment in sources.list.
  * Create an etc/apt/preferences file Pin-ing the PPA at 550 when there's
    one.
  * Gross hack to reinstall the moblin-ppa-keyring package after the trusted
    db is put back in place since otherwise we miss the GPG key we wanted.
  * Fix computation of Origin fields for PPAs named "ppa" and document the
    exception along with the expected format for the PPA var in comments.
    LP: #434243
  * Move the code taking a backup of the APT trusted keyring to just after
    sources.list generation (this is safe since there's no apt call inbetween)
    and move the installation of the moblin-archive-keyring just before the
    backup is taken; this allows dropping the hack of reinstalling the keyring
    package at the end of the build. It might break apt-get update for UMR
    before the host keys are appended to the trusted keyring though.
  * Remove ${ROOT}var/cache/apt/{,src}pkgcache.bin after installing the moblin
    keyring package, workaround for LP #442082.
  * Run apt-get update one more time after installing the moblin keyring
    package, to promote Release.gpg from APT's lists/partial/ to lists/.
    Ideally APT would do that for us on apt-key updates.
  * Also change computation of Origin field for PPAs when generating the final
    APT config; ideally we wouldn't have to duplicate this code though;
    perhaps the removal of APT preferences file can be limited to hardy too.
    Also add an Explanation: header in the final preferences.

 -- Loic Minier <email address hidden> Sun, 04 Oct 2009 13:53:40 +0200

The next successful build starting tomorrow should have these changes.

So I thought livecd-rootfs was updated nightly on the livefs buildd, but the build from tonight was using 0.97 instead of 0.99 (and failed).