USBD thing to be done (TODO)

[kuldeepdhaka]

• support for sending resume event to host
• support for power down
• test mode for USB (J Mode, K Mode, SE0 NAK, PACKET_MODE)
• Pass SOF number to SOF callback (We need to talk on this)
• Big endian support
• Generate configuration descriptor just using the number of bytes available in control buffer.
  build the descriptor on every callback of data send.
  This is going to be a tricky work but do-able.
  Before something like this get introduced, application can override GET_DESCRIPTOR (CONFIGURATION) and pass on a prebuild from flash (that is exactly like the one will be generated on fly).
• Convert usbd_transfer_status to a string name and string description. like libusb_error_name() and libusb_strerror()
• reject any submitted URB if device is disconnected.
• Unaligned memory access can be problem on platform like EFM32HG (which has a M0+).
• Remove prototype https://github.com/insane-adding-machines/unicore-mx/blob/master/include/unicore-mx/usbd/class/msc.h#L89 (less over from previous code)
• inputs in USBD add core-power and Vbus handle #50 (comment)