fix tftpboot test on boards without autoboot, swap wan/lan

This is for MIPS.  I spent a while investigating why the second virtio
net device doesn't function in qemu mips malta u-boot, but with no
success. Use the first one instead.

tests/tftpboot/script.expect

@@ -1,10 +1,10 @@
 set timeout 30
 
 spawn socat unix-connect:vm/console -
-expect "stop autoboot"
-send "\r"
-expect "=>"
-send "setenv ethact eth1\r"
+expect {
+   "stop autoboot" { send "\r" }
+   "=>" { send "\r" }
+}
 set fh [open "result/boot.scr"]
 while {[gets $fh line] >= 0} {
       expect "=>"