Allow Selection of _bleio by Board, Fixup CIRCUITPY_BLEIO Flags

[eightycc]

Allows selection of _bleio impementation by board within a port. Makes explicit HCI device inclusion by replacing use of vpath.

Changes meanings of CIRCUITPY_BLEIO* flags as follows:

• CIRCUITPY_BLEIO: Any _bleio implementation is part of this build. Automatically set.
• CIRCUITPY_BLEIO_NATIVE: Selects the port's native _bleio implementation.
• CIRCUITPY_BLEIO_HCI: Selects the serial HCI implementation.

[eightycc]

Marked as work in progress. Using CI to verify correct builds for all ports/boards.

[dhalbert]

@eightycc I was going to do this and then saw you already did! It just needs a merge conflict fixed.

I would also like to get the Module Support Matrix to include an annotation that describes which _bleio is actually supported. This PR will make that easier: we can add entries to ADDITIONAL_MODULES in docs/shared_bindings_matrix.py like:

    "_bleio (native)": "CIRCUITPY_BLEIO_NATIVE",
    "_bleio (HCI co-processor)": "CIRCUITPY_BLEIO_HCI",

I also had another thought, which I'm not sure is a good idea at all, but is an idea. Asking @tannewt also:

Suppose _bleio for HCI was actually called _bleio_hci. The libraries would do something like:

try:
    import _bleio
except ImportError
    import _bleio_hci as _bleio

Conceivably, one could make a build that would support both, though it's not of much use (though there are compile issues about that, so you don't have to duplicate the shared-bindings code).

[eightycc]

Like Barliman Butterbur in LOTR, I completely forgot about this one. One thing drives out another! I'll pick it up again, if you'd like.