lib_generic: gunzip: New function zunzip

Separate gunzip in

gunzip: Find the end of the header and call zunzip.
zunzip: Inflate gunzip block without header.

UBI fs blocks can be compresed in lzo, zlib or no-compression. The
current implementation of u-boot supported all the compressions but
there was a bug in the implementation of the zlib blocks.

UBIFS's Zlib blocks do not have header but they were compressed using
gunzip, a function used to decompress gunzip files/sectors with a
header.

This patch adds a new function zunzip that uncompress a zlib block with
no header.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@uam.es>