1. The information of /proc/mounts as below
root@qemumips:~# cat /proc/mounts |grep nfs
192.168.7.1:yyy/ / nfs rw,relatime,vers=2,rsize=4096,wsize=4096,namlen=255,hard,nolock,proto=udp,port=3049,timeo=11,retrans=3,sec=sys,mountaddr=192.168.7.1,mountvers=1,mountport=3048,mountproto=udp,local_lock=all,addr=192.168.7.1 0 0
nfsd /proc/fs/nfsd nfsd rw,relatime 0 0
root@qemumips:~# 

2. The root_is_nfs() in /etc/udhcpc.d/50default as below

root_is_nfs() {
	grep -qe '^/dev/root.*\(nfs\|smbfs\|ncp\|coda\) .*' /proc/mounts
}

3. root_is_nfs alway returns false

root@qemumips:~# grep -e '^/dev/root.*\(nfs\|smbfs\|ncp\|coda\) .*' /proc/mounts 
root@qemumips:~#