ARM: OMAP: Add omap3 USB loader tool ...

The OMAP3 supports uploading the first stage bootloader via USB.
The ROM leaves the MUSB controller enabled and it can then be used
to upload a 2nd stage image. This patch adds the omap3-usb-loader tool
and the necessary barebox support to upload the 2nd stage image.

The omap usb loader tool is downloaded from https://github.com/grant-h/omap_loader
and changed to also accept CHSETTINGS images.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>