poky.conf: Use weaker assignment for PREMIRROR

This is to facilitate distros using poky as reference
so that they can use ?= and provide an option for their
users to override it if desired

(From meta-yocto rev: cb3308d125f755cbece03d1ee00d8e255941fe9c)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>