The base_version_less_or_equal() will raise errors if
PREFERRED_VERSION_linux-yocto is None. For example, when we build
DISTRO = "nodistro", PREFERRED_VERSION_linux-yocto is not be defined
since it is defined in poky.conf, and then bitbake will
choose the higher version which is 4.8 currently, so set
PREFERRED_VERSION_linux-yocto to 4.8, otherwise, runqemu can't boot it.
Signed-off-by: Robert Yang <liezhi.yang@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
