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>

Makefile

@@ -183,6 +183,7 @@ OBJS := $(addprefix $(obj),$(OBJS))
 LIBS  = lib/libgeneric.o
 LIBS += lib/lzma/liblzma.o
 LIBS += lib/lzo/liblzo.o
+LIBS += lib/zlib/libz.o
 LIBS += $(shell if [ -f board/$(VENDOR)/common/Makefile ]; then echo \
 	"board/$(VENDOR)/common/lib$(VENDOR).o"; fi)
 LIBS += $(CPUDIR)/lib$(CPU).o