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Bootloader progress: all of the crash exception vectors in the application now set a status code in backup RAM which is passed to the bootloader, so the bootloader can tell if it was invoked as a result of an application crash, a warm reset at the application's request, a power cycle, or a request by the application to enter DFU mode.

If the application crashes, the bootloader defaults to entering DFU mode immediately (I might add a timeout counter in the future where a few crashes are allowed, but the goal is to avoid a crash loop). After a new, hopefully less buggy firmware is flashed the bootloader clears the fault and boots it. Since there's no backup battery, a full power cycle of the system will also allow restarting the crashy firmware build should you so desire.

I think I'm at a pretty good stopping point for the bootloader itself now. Next step will be adding SFTP server support to my SSH stack on the main MCU so I can actually get a binary to push to said bootloader.