Add function to generate uthash after write

Add function to generate uthash after write

uthash are generated only when a zck is read when index is parsed. In
case a comparison is done between a zck and a generated zck, to search
for hash both need to have a uthash database, so add function to create
the database when zck is created.

Add a function to assembly a zck with chunks from another zck (source).
The resulkting zck will contain all chunks from source when the hash
(compressed or not compressed in case the compressors are different)
matches.

Signed-off-by: Stefano Babic <sbabic@denx.de>