Update USB boot tools

* update tools/get_uec_image.sh for precise and proxy operation
* add cleanup to tools/warm_apts_and_pips_for_uec.sh, remove pip download
* run tools/warm_apts_and_pips_for_uec.sh as root
* update tools/build_ues_ramdisk.sh for precise; add exec sudo
* update tools/build_usb_boot.sh to use $DEST

Change-Id: If3cc0a8973805f0376dfce2bcbb75082f2e039d3