[PATCH v2] imx: Save boot source into env during init

[Marc Reilly]

Hi,

This series cleans up the imx_boot_save_loc function and changes the values
of the exported $boot_loc variable to be more consistent with the IC docs, to 
avoid potential confusion (when say booting off an SPI eeprom, $boot_loc is
now "expansion" rather than "mmc")

Cheers,
Marc