zlib: split up to match original source tree

While looking to upgrade to zlib-1.2.5, the current mondo merge of
multiple files into a single was making things way more difficult
than it should have been.  Hard to pick out what has been changed
to port it to U-Boot, been removed as useless, and bug fixes added
after the fact.

So split the single file up into the original file names, and merge
non-essential changes back from the original tree (for some reason,
style in code in a bunch of places was changed to U-Boot style even
though this isn't "U-Boot" code).

The original build style is retained -- we have a single zlib.c that
includes all the other files, and that is the only file we compile.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>