poky.conf: Define SDK_VERSION so DISTRO_VERSION overrides are supported

poky.conf: Define SDK_VERSION so DISTRO_VERSION overrides are supported

If DISTRO_VERSION is defined using an override, e.g., by defining it
in auto.conf as DISTRO_VERSION_forcevariable, then the unoverridden
value was still used in SDK_VERSION since it was defined using the :=
operator. However, there should be no reason to define SDK_VERSION
this way.

Also use getVar to access the variable to ensure tracebacks from problems
are less confusing.

[RP: Tweaked commit message]
(From meta-yocto rev: 74f43fb82a93eecfe7519a87fdf1cb0152d56b48)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>