omap3: cm-t3517: enable 'netretry' and setup timeout

SBC-T3517 evaluation board has two Eth interfaces.
Enable network retry of another interface if the default if failed
or disconnected.

Add 'netretry=yes' in the default env. Setup relevant
timeout values in the board config file.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>