USB host boot mode

USB host boot is available on Raspberry Pi 3B, 3B+, 3A+, and 2B v1.2 only. Raspberry Pi 3A+ only supports mass storage boot, not network boot.

The USB host boot mode follows this sequence:

Enable the USB port and wait for D+ line to be pulled high indicating a USB 2.0 device (we only support USB2.0)
If the device is a hub:
- Enable power to all downstream ports of the hub
- For each port, loop for a maximum of two seconds (or five seconds if program_usb_boot_timeout=1 has been set)
  - Release from reset and wait for D+ to be driven high to indicate that a device is connected
  - If a device is detected:
    - Send "Get Device Descriptor"
      - If VID == SMSC && PID == 9500
        
        Add device to Ethernet device list
    - If class interface == mass storage class
      - Add device to mass storage device list
Else
- Enumerate single device
Go through mass storage device list
- Boot from mass storage device
Go through Ethernet device list
- Boot from Ethernet

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