Fix erase timeouts in some bootloaders and atxmega16c4 follow-up

There are two items in this request. The main one is to improve the behaviour of erase on some chips where the bootloader returns a busy status rather than waiting for the erase to complete before returning status.

The other is a follow-up to Olivier Pisano's pull request which added support for the atxmega16c4. This corrects the bootloader size and adds the sister chip atxmega32c4. It also adds these to the documentation.