This reverts commit f0aec23a91368e916a53e6072f2173bb481b1544.

Asias He writes:

  'ip=dhcp' makes kernel try to use nfs as root fs which results in a long
  time 'wait and fail' and finally fallback to 9p as root fs.

  [    1.688000] Sending DHCP requests ., OK
  [    1.695000] IP-Config: Got DHCP answer from 192.168.33.1, my address
  is 192.168.33.15
  [    1.699000] IP-Config: Complete:
  [    1.700000]      device=eth0, addr=192.168.33.15, mask=255.255.255.0,
  gw=192.168.33.1,
  [    1.703000]      host=192.168.33.15, domain=stu.buaa.edu.cn,
  nis-domain=(none),
  [    1.706000]      bootserver=192.168.33.1, rootserver=192.168.33.1,
  rootpath=
  [   36.769000] VFS: Unable to mount root fs via NFS, trying floppy.
  [   36.771000] VFS: Mounted root (9p filesystem) on device 0:14.