This pulls out all the common header definitions for the F1, L1, F4 and F37x
parts. It's verified against the datasheet for F2 as well, but we don't have
any good F2 test boards or any support for that yet. (The F2 header would be
_exactly_ the same as the F4 header, so it's a target for a future round of
unification, not this one)
Tested with f1, f4 and l1 examples from the examples repository.
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