Technically I updated to a development version somewhere between 17.6.3
and 17.6.4, but I don't doubt its stability.

This is a good update because I'd like users of the library to be
confident that it works with the latest version of ChibiOS.
Interestingly, none of the changes I made were in the library code, so
it did before, but now I know for sure.

I'm unclear as to whether I can now get rid of the INT_N thread in favor
of a pin interrupt.  I'll have to see if I can, or if that will have to
wait for still more ChibiOS updates.

After much poking around, I finally figured out how to make this work!
The problem was that the ChibiOS initialization code had disabled
interrupts before the __early_init function was run, making the
bootloader not work despite my best intentions.  This was fixed by
enabling interrupts just before jumping to the bootloader.  This in turn
required changing to GNU-flavored C11 to include an inline assembly
instruction.

That's the name of the board now, so the configuration should match.