CHROMIUM: md: dm-verity convert to upstream version

CHROMIUM: md: dm-verity convert to upstream version

This CL only has the upstream version of dm-verity.c.
It was take from the 3.4.22 version of linux.
I've also removed dm-verity.h and dm-bht.c which are not
part of the upstream version of verity.
Removed the old documenation and added the new documents.
The next CL will have have the changes to make it work.

BUG=chromium-os:31803
TEST=platform_DMVerityCorruption, platform_DMVerityBitCorruption

Change-Id: Ic7050bc2609ef322080f76286a1f41417dc7def8
Signed-off-by: Paul Taysom <taysom@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/41206