bloblist: Use explicit numbering for the tags

At present if someone adds a tag in the middle of the list it works well
enough within a U-Boot build. But if these tags are used in another
project, or with an older version of SPL, the numbers make become
inconsistent.

Use explicit tag numbers that never change, to resolve this problem.
Allocate areas for existing U-Boot tags and set up an area for use by
projects and vendors, as well as for private use. Keep tags above
0x10000 unallocated for now.

Update bloblist_tag_name() and the tests to work with this new setup.

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