imx: Save boot source into env during init

[Marc Reilly]

Hi,

This series saves the boot location into an environment variable $boot_loc for 
use in init/boot scripts etc. on imx25 and imx35.

Could someone comment as to whether the first patch is strictly necessary? 
- env_push_context() needs to be called before imx_boot_save_loc() (which 
calls export()), and I'm not sure whether it would always be guaranteed 
if they are both late_initcalls. I assume it comes down to linking order..

Cheers,
Marc