version: Keep the maintenance version defined and use it if needed

 * The maintenance version should not be set optionally
    to its official value status, but only be used when needed

 * Use the TARGET_VENDOR_SHOW_MAINTENANCE_VERSION := true
    introduced in I77c924a13122c8e1d9af14501e46bc532f7aefbd
    to show the value of the maintenance version in releases,
    with an intermediary CM_VERSION_MAINTENANCE variable

Change-Id: I22b944a1d2d8586e5f3c75e19ca1c53aa3b22b59