sandbox: README: use setenv ethrotate no

If we want to control which network interface is actually used, we have to
issue 'setenv ethrotate no'. If ethrotate is not set any interface may be
used.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/sandbox/README.sandbox

@@ -248,6 +248,7 @@ DHCP
 ....
 
 setenv autoload no
+setenv ethrotate no
 setenv ethact eth1
 dhcp
 
@@ -255,6 +256,7 @@ PING
 ....
 
 setenv autoload no
+setenv ethrotate no
 setenv ethact eth1
 dhcp
 ping $gatewayip
@@ -263,6 +265,7 @@ TFTP
 ....
 
 setenv autoload no
+setenv ethrotate no
 setenv ethact eth1
 dhcp
 setenv serverip WWW.XXX.YYY.ZZZ
@@ -287,6 +290,7 @@ operation being tested on the lo interface.
 TFTP
 ....
 
+setenv ethrotate no
 setenv ethact eth5
 tftpboot u-boot.bin