zlib: add gunzip() support ...

This commit adds some more files to use zlib from TF.

To use zlib, ->zalloc and ->zfree hooks are needed.  The implementation
depends on the system.  For user-space, the libc provides malloc() and
friends.  Unfortunately, ARM Trusted Firmware does not provide malloc()
or any concept of dynamic memory allocation.

I implemented very simple calloc() and free() for this.  Stupidly,
zfree() never frees memory, but it works enough for this.

The purpose of using zlib is to implement gunzip() - this function
takes compressed data from in_buf, then dumps the decompressed data
to oub_buf.  The work_buf is used for memory allocation during the
decompress.  Upon exit, it updates in_buf and out_buf.  If successful,
in_buf points to the end of input data, out_buf to the end of the
decompressed data.

To use this feature, you need to do:

 - include lib/zlib/zlib.mk from your platform.mk

 - add $(ZLIB_SOURCES) to your BL*_SOURCES

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>