Add transitional flag for ARM kernel signing in mod_image_for_recovery.sh.
Add transitional flag for ARM kernel signing in mod_image_for_recovery.sh.

Enable the ARM kernel signing procedure in the recovery image. Will remove it
when the procedure is stable.

BUG=chromium-os:12352,chromium-os:1305
TEST=run "build_image" and "mod_image_for_recovery.sh
--crosbug12352_arm_kernel_signing" to create a recovery image.
Then use u-boot recovery mode to boot the recovery image. Successfully verify
and boot the kernel.

Change-Id: I1bf911176c6f3adaebf3aa9abc86488a48a60494

Review URL: http://codereview.chromium.org/6635010

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