This commit adds support for using Bluetooth on the unix port via a H4
serial interface (distinct from a USB dongle), with both BTstack and NimBLE
Bluetooth stacks.
Note that MICROPY_PY_BLUETOOTH is now disabled for the coverage variant.
Prior to this commit Bluetooth was anyway not being built on Travis because
libusb was not detected. But now that bluetooth works in H4 mode it will
be built, and will lead to a large decrease in coverage because Bluetooth
tests cannot be run on Travis.
