net: add 'ethrotate' environment variable

[PATCH] net: add 'ethrotate' environment variable

This patch replaces the buildtime configuration option
CONFIG_NET_DO_NOT_TRY_ANOTHER through the 'ethrotate' runtime
configuration veriable. See README.

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