Previously the interaction between the different layers of the Bluetooth
stack was different on each port and each stack. This commit defines
common interfaces between them and implements them for cyw43, btstack,
nimble, stm32, unix.
This commit ensures that the BLE stack is active before allowing operations
that may otherwise crash if it's not active. It also clarifies the state
better (adding the "stopping" state) and renames mp_bluetooth_is_enabled to
the more self-explanatory mp_bluetooth_is_active.
Move extmod/modbluetooth_nimble.* to extmod/nimble. And move common
Makefile lines to extmod/nimble/nimble.mk (which was previously only used
by stm32). This allows (upcoming) btstack to follow a similar structure.
Work done in collaboration with Jim Mussared aka @jimmo.
