targets/xorg: Install lts-saucy drivers on precise for Haswell support

We first pull in `lts-saucy` replacement drivers, then `xorg`:
- Doing it in that order still pulls in `xorg` dependencies like
  `xterm` and `xinit`, and avoids the need to specify them manually.
- Removing ``--minimal`` from the `lts-saucy` line also works, but
  also pulls in stuff like an updated kernel, which we don't want.
- Installing `xorg` after `lts-saucy` does not install old drivers
  (so there is no useless download).
- On existing chroots, installing `lts-saucy` uninstalls old drivers,
  and marks a number of packages as not required, like `xterm` and
  `xinit` (which would be cleaned later by autoclean). Reinstalling
  `xorg` does not install new packages, but makes sure these packages
  are not autocleaned.

Tested:
 - Current HEAD installer, `-t xorg`, then update with this branch: no important package is added or removed.
 - Fresh install with this branch: installed packages are same as case above.
 - On `peppy`, direct switching between Chrome OS and crouton works with `lts-saucy` drivers (fixes #754, #870, #884 and a few others).

Also tested with the (not yet mature) x11 test infrastructure, formatted output here: http://drinkcat.github.io/chroagh/2014-06-17_01-22-27_drinkcat_chroagh_x11test.tmp-add-saucy-lts_35.html . The important bit here is that we have OpenGL acceleration in `precise` on all x86 platforms (fixes #639).