cmd: blkmap: Add blkmap command

Add a frontend for the blkmap subsystem. In addition to the common
block device operations, this allows users to create and destroy
devices, and map in memory and slices of other block devices.

With that we support two primary use-cases:

- Being able to "distro boot" from a RAM disk. I.e., from an image
  where the kernel is stored in /boot of some filesystem supported
  by U-Boot.

- Accessing filesystems not located on exact partition boundaries,
  e.g. when a filesystem image is wrapped in an FIT image and stored
  in a disk partition.

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