cros_make_image_bootable: default to verity_depth=0
cros_make_image_bootable: default to verity_depth=0

Verity only supports a depth of 0. Passing anything other than 0 will
cause a verity failure.

BUG=14314
TEST=Verified that verity does not fail when do depth is passed.
     Before this change, verity would fail with the error message
     reported in the bug.

Change-Id: I68495c4486284744ee0779b6c9cdc0ef214519c7

R=gauravsh@google.com,scottz@google.com,drewry@google.com

Review URL: http://codereview.chromium.org/6873134

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