This changes storage.mount() to require that a mount point exist
on the parent file system.
A bug in background tasks is also fixed where the function
parameter is cleared on pending callbacks during "reset".
Disk usage is shown on the directory listing and changes based on
the mounted file system. Writable is also loaded per-directory.
Fixes#8108. Fixes#8690. Fixes#8107.
Unify USB-related makefile var and C def as CIRCUITPY_USB.
Always define it as 0 or 1, same as all other settings.
USB_AVAILABLE was conditionally defined in supervisor.mk,
but never actually used to #ifdef USB-related code.
Loosely related to #4546
