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: 
[] (dump_backtrace+0x0/0x110) from [] (dump_stack+0x18/0x1c)
 r7:c5bf8000 r6:00000000 r5:c002dbfc r4:c0551710
[] (dump_stack+0x0/0x1c) from [] (panic+0x60/0x17c)
[] (panic+0x0/0x17c) from [] (mount_block_root+0x1e0/0x220)
 r3:00000000 r2:00000000 r1:c5825f58 r0:c0485b7c
[] (mount_block_root+0x0/0x220) from [] (mount_root+0xac/0xcc)
[] (mount_root+0x0/0xcc) from [] (prepare_namespace+0x170/0x1d4)
 r4:c0550fa4
[] (prepare_namespace+0x0/0x1d4) from [] (kernel_init+0x114/0x154)
 r5:c0008670 r4:c0550f40
[] (kernel_init+0x0/0x154) from [] (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！

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