采用TFTP 下载内核启动挂载网络文件系统方式出现VFS: Unable to mount root fs via NFS, trying floppy.

目的：tftp下载内核，并且通过NFS挂载根文件系统出现以下错误

虚拟机IP：192.168.253.128      板子IP:192.168.253.183

/etc/exports 内容：/opt/DVRRDK_02.00.00.23/target/rfs *(rw,nohide,insecure,no_subtree_check,async,no_root_squash)

IP-Config: Complete:

     device=eth0, addr=192.168.253.183, mask=255.255.255.0, gw=192.168.253.1,

     host=192.168.253.183, domain=, nis-domain=(none),

     bootserver=192.168.253.142, rootserver=192.168.253.142, rootpath=

nfs相关设置信息截图：

虚拟机的防火墙也关闭了，启动延迟设置为8秒，内核配置也选择了ROOT FILE SYSTEM ON NFS选项

.config文件开启选项截图为

错误提示：

ata2: EH complete

ata1: EH complete

VFS: Unable to mount root fs via NFS, trying floppy.

VFS: Cannot open root device "nfs" or unknown-block(2,0)

Please append a correct "root=" boot option; here are the available partitions:

1f00            2432 mtdblock0  (driver?)

1f01             128 mtdblock1  (driver?)

1f02            3072 mtdblock2  (driver?)

1f03            4352 mtdblock3  (driver?)

1f04          252160 mtdblock4  (driver?)

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)

Backtrace: 

[<c0048b90>] (dump_backtrace+0x0/0x110) from [<c03c3564>] (dump_stack+0x18/0x1c)

 r7:c5bf8000 r6:00000000 r5:c002dbfc r4:c0551710

[<c03c354c>] (dump_stack+0x0/0x1c) from [<c03c35c8>] (panic+0x60/0x17c)

[<c03c3568>] (panic+0x0/0x17c) from [<c0009254>] (mount_block_root+0x1e0/0x220)

 r3:00000000 r2:00000000 r1:c5825f58 r0:c0485b7c

[<c0009074>] (mount_block_root+0x0/0x220) from [<c0009340>] (mount_root+0xac/0xcc)

[<c0009294>] (mount_root+0x0/0xcc) from [<c00094d0>] (prepare_namespace+0x170/0x1d4)

 r4:c0550fa4

[<c0009360>] (prepare_namespace+0x0/0x1d4) from [<c0008784>] (kernel_init+0x114/0x154)

 r5:c0008670 r4:c0550f40

[<c0008670>] (kernel_init+0x0/0x154) from [<c006ef78>] (do_exit+0x0/0x5e4)

 r5:c0008670 r4:00000000

附件为板子启动的全部信息，希望高手指教！