this fixes a regression from 4.2 to 4.3.
previously we automatically set the diagnoals to 1,1,1 if they were
0,0,0. We don't do that any more. I was helping a user who had copied
an old config with 0,0,0 for diagonals and did not understand two
things:
- the compass did not work in 4.3
- large vehicle mag cal didn't work
