installer/ubuntu/prepare: detect_mirror function

Detects the current mirror.
targets/xorg: >=saucy, kernel 3.4, pin down precise mesa drivers.

On saucy onwards, if kernel version is 3.4, manually pin down old
mesa libraries from precise (mesa 8), as new ones (mesa >=9)
require kernel >=3.6.

Fixes #704.
targets/xorg: Only downgrade on non-Atom Intel chipsets