Add support for "keydevices" in TRESOR.

This allows you to use an arbitrary block of a block device as an
additional salt to TRESOR.

It uses the tresorkeydevice= parameter and name_to_dev_t (ie the
same semantics as root=) to select the block device.

Though the block number should be considered private data, no
effort is made to keep it out of memory.  This should be fixed.

The read uses a simple bio, so many block devices are likely to
cache the request, resulting in parts of your key ending up in
memory.  This might also be fixed, though it would likely require
significant effort.  If you use a regular block device as your key
device, you are likely safe.  (TODO: check that that is actually
true)

Note that you absolutely need to include all modules in your kernel
which are required to connect the keydevice.  If you fail to do so,
your boot will hang.