Use platform_family instead of platform

This will allow the ibm_powerkvm platform to be recognized and will
simplify uses of multiple platforms from the same family.

Change-Id: Iebf2aad9f3b4649fb5c27d2fc93ee60c67e42737
Partially-Implements: blueprint add-ibm-powerkvm-enablement
Implements: blueprint platform-family
Merge "Use platform_family instead of platform"