ti: am335x/am437x/omap5 devices: Unify network environment variables

Across several devices network environment variables are duplicated.
Move these variables to a common include file which insures the environment
variables are reused and insures devices across product lines share the same
values.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>