mod_image_for_recovery.sh: some cleanup.
mod_image_for_recovery.sh: some cleanup.

Motivated by the need to share the default recovery image name across
scripts (image_to_usb.sh in this case) and have it setup in common.sh.
Includes some general cleanup steps such flags and usage strings,
migrating function definitions to the beginning of the script, etc.

BUG=chromium-os:26010
TEST=Tested mod_image_for_recovery.sh on local host.

Change-Id: Ife3731915302a2e7be8beb750501d0ae9a0c43a4
Reviewed-on: https://gerrit.chromium.org/gerrit/15523
Reviewed-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

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