blk: blkmap: Add basic infrastructure

blkmaps are loosely modeled on Linux's device mapper subsystem. The
basic idea is that you can create virtual block devices whose blocks
can be backed by a plethora of sources that are user configurable.

This change just adds the basic infrastructure for creating and
removing blkmap devices. Subsequent changes will extend this to add
support for actual mappings.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Simon Glass <sjg@chromium.org>