cmake: explain BUILD_VERSION and KERNEL_VERSION_*

- Add examples for the latter.
- Point at each other and highlight how independent they are from each
  other.
- State their inputs and outputs in plain English.
- Fix "git describe" error message giving the wrong impression that
  everyone cares about BUILD_VERSION. Only the boot banner cares now.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>