OMAP: networking support for SPL

This patch adds support for networking in SPL. Some devices are
capable of loading SPL via network so it makes sense to load the
main U-Boot binary via network too. This patch tries to use
existing network code as much as possible. Unfortunately, it depends
on environment which in turn depends on other code so SPL size
is increased significantly. No effort was done to decouple network
code and environment so far.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@ti.com>