qemu usage:

  # ifconfig br0:1 172.20.0.1
  # qemu-system-mips -nodefaults -nographic -M malta -m 256 \
    -serial stdio -monitor null \
    -bios ./barebox-flash-image \
    -net nic,vlan=0,model=rtl8139 \
    -net tap,vlan=0,script=net_br0.sh

  ...

  barebox:/ eth0.ipaddr=172.20.0.2
  barebox:/ eth0.serverip=172.20.0.1
  barebox:/ ping 172.20.0.1
  phy0: Link is up - 100/Full
  host 172.20.0.1 is alive
  barebox:/

Sample net_br0.sh (for connecting to br0 interface, so you can
connect barebox to your real network):

  #!/bin/sh

  brctl addif br0 $1
  ifconfig $1 up

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>