zlib: remove historical movement of libz.so to /lib

Historically there's been a split between /lib for early boot and
/usr/lib for everything else, but with modern systems this split is
meaningless and incomplete. If a minimal system for early boot is
needed, it should be a full minimal system in a initramfs.

[RP: Fixed up selftest to match]
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>