rpi: Add USB boot support (#440)

Documentation/boards/raspberrypi.md

@@ -2,7 +2,7 @@
 
 Supported Hardware:
 
-| Device | Board | 
+| Device | Board |
 |--------|-----------|
 | Raspberry Pi A+/B/B+| rpi |
 | Raspberry Pi Zero | rpi |
@@ -21,7 +21,7 @@ For access to terminal over serial console, add `console=ttyAMA0,115200` to `cmd
 
 ## I2C
 
-Add `dtparam=i2c1=on` and `dtparam=i2c_arm=on` to `config.txt`. After that we create a module file on host with [config usb stick][config] or direct into `/etc/modules-load.d`. 
+Add `dtparam=i2c1=on` and `dtparam=i2c_arm=on` to `config.txt`. After that we create a module file on host with [config usb stick][config] or direct into `/etc/modules-load.d`.
 
 rpi-i2c.conf:
 ```
@@ -29,6 +29,19 @@ i2c-dev
 i2c-bcm2708
 ```
 
+## USB Boot
+
+USB mass storage boot is available on Raspberry Pi 3B, 3B+, 3A+, and 2B v1.2.
+To enable USB boot, add `program_usb_boot_mode=1` into `config.txt`. Note that this **permanently** alters the one-time programmable memory of the device.
+
+For more information see [RaspberryPi](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md).
+
+### Caveats
+
+* All bootable SD cards must be removed.
+* Boot time can be significantly longer with USB. This is due to the boot process first attempting to boot from SD card, failing, and resorting to USB.
+* Many USB drives simply do not work for boot. This is likely due to minimal driver support in uboot and will not be fixed. If you can't get it to boot on one drive, try a different brand/model. SanDisk Cruzer drives seem to have a higher rate of issues.
+
 ## Tweaks
 
 If you don't need bluetooth, disabled it with add `dtoverlay=pi3-disable-bt` into `config.txt`.