arm: am57xx: cl-som-am57x: update default env

Modify U-Boot default env settings.

Boot sequence:
1. SD card boot script
2. SD card boot no script
3. SATA boot script
4. SATA boot no script
5. eMMC boot script
6. eMMC boot no script

Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>