default-versions.inc: Make PREFERRED_VERSION_openssl* overwritable
defaultsetup.conf and therefore default-versions.inc is sourced pretty late in bitbake.conf.
default-versions.inc overwrites previous assignments of PREFERRED_VERSION_openssl.
Assigning it with ?= allows other .conf files to assign correctly.
in particular assignment in conf/local.conf and machine config is used instead of default.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
