imx53: support load of single file from command line

[olafmandel]

There is some problem with the iMX53 (maybe also with others?) that
requires loading *.imx files in multiple steps: first the DCD, then load
only the binary without the header without running it and then load
everything and run it. This required adding the files to be loaded into
the configuration file, with three lines:

foo.imx:dcd,plug
foo.imx:load <addr>,skip <header-length>
foo.imx:jump header2

This adds a command line option "-w" that can be used to load such a
file with one simple command:

imx_usb foo.imx -w