linux-info.eclass: import from main tree.

This version includes the fix for building from a host running a stable
kernel like 2.6.38.1 without having kernel sources available.

Change-Id: Ie32a2faae9a8d2567d21711eef79ebfaad6e204c

BUG=chromium-os:13616
TEST=./setup_board and ./build-packages --nousepkg for arm-generic and x86-generic

Review URL: http://codereview.chromium.org/6792005