seco-mx6: adaptation for edgehog
This is part of the fix for: https://secogroup.atlassian.net/browse/YT-91 i.MX6: Edgehog: Integrate the SAN* devices into edgehog seco-imx layer
Following changes were done in u-boot:
- added CONFIG_EDGEHOG_OS, as it used to determine that we are booting Edgehog image.
- added a separate config for edgehog based images: seco-mx6_edgehog_defconfig
- switched seco-mx6 env to use boot command from MX8
- fixed issues with current env, to make it work as intended
- and some cleanup