net: Use env callbacks for net variables

Instead of checking for changes to the env each time we enter the
net_loop, use the env callbacks to update the values of the variables.
Don't update the variables when the source was programmatic, since the
variables were the source of the new value.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>