net: Add CONFIG_NET_DO_NOT_TRY_ANOTHER option

When CONFIG_NET_DO_NOT_TRY_ANOTHER is defined U-Boot's
networking stack does not automatically switch to
another interface. This patch does not touch the default
behavior.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>