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>

meta-yocto/conf/distro/poky.conf

@@ -45,7 +45,7 @@ TCLIBCAPPEND = ""
 QEMU_TARGETS ?= "arm i386 mips mipsel ppc x86_64"
 # Other QEMU_TARGETS "mips64 mips64el sh4"
 
-PREMIRRORS ?= "\
+PREMIRRORS ??= "\
 bzr://.*/.*   http://downloads.yoctoproject.org/mirror/sources/ \n \
 cvs://.*/.*   http://downloads.yoctoproject.org/mirror/sources/ \n \
 git://.*/.*   http://downloads.yoctoproject.org/mirror/sources/ \n \