cmake: check sdk_version format

Encountered situation when sdk_version string was empty
(as the result of corrupted installation).

The version should had 0.0.0 format.
Patch add check for this and descriptive failure message.

Objective is to help the user to recognize the installation failure.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>