Fix clean_loopback_devices to actually parse board parameter
Fix clean_loopback_devices to actually parse board parameter

clean_loopback_devices was never parsing command line arguments or
properly setting the default board. As such it would always use the
x86-wayland overlay as it came last alphabetically in the directory
listing.

BUG=chromium-os:22671
TEST=Ran clean_loopback_devices with various --board arguments
  Validated correct OUTPUT_DIR values for those, including an error
  and abort on null

Change-Id: Ia8826bbeb88ed5bdac0aa7698dd262f72de2749a
Reviewed-on: https://gerrit.chromium.org/gerrit/11332
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Ready: Jon Kliegman <7addd549d0b04b2dc663a0a7eb2c1810caf11d96@chromium.org>
Tested-by: Jon Kliegman <7addd549d0b04b2dc663a0a7eb2c1810caf11d96@chromium.org>

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