The board API should only ask for a UsbBus

[ia0]

This simplifies board implementations by having USB handling fully in the scheduler, thus shared and common across all boards.

This relates to #60 and #61.

The design could be as follow:

• Add an api-usb-bus feature for usb::Api::Bus.
• Add an internal-no-api-usb-bus as a dependency of all other USB APIs.
• Make sure at compile time that api-usb-bus and internal-no-api-usb-bus are not enabled at the same time.
• Design the applet API to configure and use the USB bus (protocol, serial, HID, HID CTAP, etc).