USB downloader and USB second stage bootloader for OMAP44xx

aboot.bin

aboot.bin is a designed to be a second stage USB bootloader (what TI
calls x-loader) for the USB peripheral boot mode of OMAP44xx.

Currently it's somewhat pandaboard-centric, but that should be
easy enough to clean up -- it shouldn't touch any gpiomux config
except (maybe) for the UART.

All it does is send a u32 message (0xaabbccdd) back over the USB link,
then reads a u32 size from the host. Then it downloads size bytes
from the host to 0x82000000 and jumps to that address.

usbboot

usage: usbboot <2ndstage> [ ]

• usbboot will poll every 250ms until it locates an OMAP device
  (VID 0451, PID d00f)
• it then will send the 2ndstage binary to the device
• if an image was also provided, it will wait for the 2ndstage to
  send the go-ahead response (0xaabbccdd) and then send that image