image_to_usb.sh: prompt when detected image is not the default image.
image_to_usb.sh: prompt when detected image is not the default image.

Having introduced autodetection and autoselection of images, some users
may be surprised to find that the script will happily choose to copy an
image other than chromiumos_image.bin when only one image is present.
Since this is substantially different from the previous behavior, we now
present a menu (with a single choice) in this case as well.

Also fixing the list of images to be detected and properly prioritizing
a default image over others. The code should already work with the
future image naming scheme, where chromiumos_image.bin will be a symlink
to one of six concrete images (with unique names).

BUG=None
TEST=Ran script with different image configurations and flags.

Change-Id: I5d448c5425754496d256258281694cd7ec2554d2
Reviewed-on: https://gerrit.chromium.org/gerrit/15982
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Reviewed-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

crawford/scripts,gtank/scripts,coreos/scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,mischief/scripts,vmware/coreos-scripts,gtank/scripts,coreos/scripts,sigma/coreos-scripts,trnubo/scripts,BugRoger/scripts,BugRoger/scripts,glevand/coreos--scripts,glevand/coreos--scripts,nicescale/scripts,trnubo/scripts,mjg59/scripts,crawford/scripts,fivethreeo/scripts,zhang0137/scripts,mischief/scripts,mjg59/scripts,marineam/coreos-scripts,trnubo/scripts,vmware/coreos-scripts,mjg59/scripts,sigma/coreos-scripts,fivethreeo/scripts,smilart/scripts,andrejro/coreos--scripts,nicescale/scripts,bcwaldon/coreos-scripts,endocode/scripts,fivethreeo/scripts,sigma/coreos-scripts,BugRoger/scripts,gtank/scripts,crawford/scripts,vmware/coreos-scripts,endocode/scripts,zhang0137/scripts,zhang0137/scripts,mischief/scripts,andrejro/coreos--scripts,smilart/scripts,nicescale/scripts,marineam/coreos-scripts,endocode/scripts,bcwaldon/coreos-scripts,smilart/scripts,marineam/coreos-scripts