Simplification and cleanup of build process

Simplification and cleanup of build process

Docker build image:
- Uplift of the base Ubuntu version to 14.04.
- Instead of building the Ubuntu image with debootstrap, we use the official
  Docker repository version of Ubuntu.
- Cleanup of the way how runcontext operates
- Fix: Make sure that the build system can be run as root

Top Makefile:
- Building the Fuel ISO on Docker local /tmp directory not to pollute local
  directory with files owned by root.
- Removed unused targets.

Change-Id: I684348b2a15546eadf98d810239a84ff9798b1e8
Signed-off-by: Stefan K. Berg <stefan.k.berg@ericsson.com>