build_image: hard-code verity_depth to 0
build_image: hard-code verity_depth to 0

Plan is to eventually deprecate depth as a configurable parameter.
depth=0 means compute the depth based on a regular trie with a
single block of hashes at the root node.

BUG=chromium-os:9752
TEST=platform_DMVerity

Change-Id: I5c5f1b1f2c2079d7e1ef6c7a55c859b463a59aee

R=wad@chromium.org,taysom@chromium.org,ups@chromium.org,gauravsh@chromium.org,jimherbert@chromium.org

Review URL: http://codereview.chromium.org/6810006

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