V2.2 - minor fixes, diagnostic tool

Just bundling a few minor tweaks that have been waiting on the main branch for a while.

Since the bootloader can't show a useful message if something is wrong, I've also added a simple diagnostic tool.

If you can't get the bootloader menu to show up, flashing and running this tool should show you what is wrong. All steps need to show "PASS" for the bootloader to be able to bring up the menu.