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Well, after a few bug fixes, here we are.

It's now a bootloader with no firmware update capability, so not super useful.

But it does boot time CRC32 verification (would be easy to swap out with a SHA, HMAC, curve25519 signature, etc if I wanted cryptographic checks, but that's not necessary for this application) to detect flash corruption, automatically updates the saved CRC if a new firmware version is loaded over JTAG, and then boots the app.

What's missing is:
* Enter firmware update flow, rather than infinite looping, if the CRC check fails
* Enter firmware update flow if application crashes (hard fault, WDT failure, etc) repeatedly
* Enter firmware update flow when requested from the main application (i.e. specific SPI command saying "enter DFU mode")
* Make said firmware update flow actually do something