-------------- omap4_usb_booting --------------

omap4 processors have support for booting from the usb port. To be able to fully
use this feature you will need to enable the following:
    OMAP4_USBBOOT
        This CONFIG_ option adds the required infrastructure.
    DRIVER_SERIAL_OMAP4_USBBOOT
        This adds console support over the same usb port used for booting.
    FS_OMAP4_USBBOOT
        This adds filesystem support over the same usb port used for booting.

To send the bootloader to the processor, execute the utility omap4_usbboot which
can be found in the scripts subdirectory.
omap4_usbboot takes two parameters:
    the bootloader image
    a directory name which will be the root for the guest system
Once omap4_usbboot is running it will wait for enumeration of the omap4 cpu on
the host usb subsystem. Then power on or reset the cpu with the correct sys_boot
or SAR memory configuration.

If everything works, barebox's first stage will boot and afterwards it will load
the second stage found at "/barebox.bin".
Barebox's second stage will still have access to the usb filesystem, so an
initrd and a zImage can be loaded.

Overall this procedure frees the developer of continously be changing the SD
card or other boot media from host to target.