verity: create a regular trie when depth=0

We want to eventually remove support for the depth constructor parameter
only create trees where level[0]->count = 1.

BUG=9752
TEST=Ran dm-verity.git unit tests. Ran platform_DMVerityCorruption on H/W.

Change-Id: Ie05ebc93a9c52055b4573ccd8811acbbb114adf3

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

Review URL: http://codereview.chromium.org/6670096