imx(25,35): save boot location into $barebox_loc env.
Saves the boot source into an environment variable so env scripts
can more easily use boot source information.
Note only tested on imx35. I haven't added support for any other variants
because I'm not familiar with them. (And can't test them anyway).
Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>