Go more into detail about the relationship of the prepare stage and the
bootloader image, and about the generated images. Also don't use
mxs-usb-loader anymore since it has been removed.

Fixes: 3ca576cc4a4236a2556ab17 "scripts: imx/mxs remove mxs-usb-loader"
Signed-off-by: Roland Hieber <r.hieber@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>