Convert Firmware Upgrade code back to qextserialport

QSerialPort buffered implementation was too slow in Windows. Now Windows flash is just slow, instead of crazy slow. Did not affect windows flash reliability which is still around 30% pass rate. But that is the strange boot loader problem is being looked at. OSX pass rate if much higher. Maybe 50-75%.

- Modified the error reporting to display the address at which flash failed, which seems to be a clue to the bootloader problem.
- Modified flash failed message to say that there is a know issue which requires trying again.