cmake: Zephyr sdk backward compatibility with 0.11.1 and 0.11.2

This commit introduces backward compatibility with Zephyr SDK 0.11.1
and 0.11.2 so that users having one of those versions installed can
continue to use that version.

This remove the need to force users to update their SDK.

This is kept in independent commit to ensure it can easily be reverted
when minimum required Zephyr SDK is bumped to version 0.12.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>