bloblist: Add a command

It is helpful to be able to see basic statistics about the bloblist and
also to list its contents. Add a 'bloblist' command to handle this.

Put the display functions in the bloblist modules rather than in the
command code itself. That allows showing a list from SPL, where commands
are not available.

Also make bloblist_first/next_blob() static as they are not used outside
this file.

Signed-off-by: Simon Glass <sjg@chromium.org>