`flashpage_write_raw()` got renamed to `flashpage_write()`.
Now `sam0_flashpage_aux_write_raw()` is the only remaining 'raw'
function, even though it behaves just like `flashpage_write()`.
So let's also rename that for consistency.
-) verify correctness of data written by raw write test
-) write to really last flash page and not last - 1
-) use flashpage_addr instead of manual calculation