imgtool: change --max-align default value

The value of `--max-align` parameter passed to imgtool can never be
less than the value of `--align` parameter. At present the default
value of `--max-align` is fixed at 8. This forces user to pass the
parameter even when its value can be safely inferred.

Change the default value of the `--max-align` parameter to the larger
of the two values: `--align` or 8. Consequently, the user is required
to pass the parameter only if the flash alignment of the primary and
secondary slot differ.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>