package/binutils: add optional support for zlib

When zlib was compiled before, binutils will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/libbfd-2.24.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libz.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Apparantly the build system offers no support to control the
dependency, --without-system-zlib with zlib installed still provides
libbfd-2.24.so linked to libz.so.1.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>