2.5.6 - Critical bugfixes and SerialUPDI improvements

This update works around a bug in the the IDE that prevented the fix for Optiboot uploads from working. It also expands the list of SerialUPDI options and improves the accuracy of the write_delay functionality. These should help people who have been having problems getting SerialUPDI to work at speeds higher than 57600 baud, which was a particular problem on Mac and Linux systems.