ti: am335x/am437x/omap5 devices: Fix breakage when CONFIG_CMD_NET is not used

Currently there is no default value for NETARGS if CONFIG_CMD_NET=y isn't set.
This results in build errors which was first discovered when trying to run
make env.

By defining a blank NETARGS these errors can be avoided.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Tested-by: Maxin B. John <maxin.john@enea.com>
Reviewed-by: Tom Rini <trini@konsulko.com>