usb/epc_usb
====
Contains device-independent USB implementation, services MAXUSB stack
and is given the actual USB descriptors & callbacks on initialization.
Handles USB events and configures/deconfigures the device accordingly
during the USB setup procedure.

usb/mass_storage & cdcacm
====
Contain device-specific implementations, service MAXUSB's device class
APIs for cdcacm and mass storage.
Here, mass storage is independent of the underlying storage device.

modules/usb
====
Public apic_usb API, configuration and storage device management.
Defines the actual USB device descriptors and services the upc_usb API.

If we want to add SD card functionality, this would be the place to add
it.