ubuntu: use kernel 3.13 (lts-trusty)

* packages/deb/debian-boot/01_mount_target_proc.sh: the linux-image
  pre-install script uses /proc/cpuinfo to query CPU features without
  checking if /proc has been mounted. This script mounts proc in /target
  before installing the kernel itself.

* packages/deb/debian-boot/initrd.mk: include the helper script for
  mounting /proc into initramfs.

* config.mk: use lts-trusty (Linux 3.13) kernel and the corresponding
  netboot variant (netboot-trusty) by default.

Change-Id: Id6a2578e36f5f303ffa05ed4d702b119964f62a4
Related-bug: #1390044
Related-bug: #1383641
Merge "ubuntu: use kernel 3.13 (lts-trusty)"