[systemd-devel] using /dev/root in fstab

[microcai]

 a long ago, /dev/root is an symblic to real root device. no matter it is on 
NFS or HDD, we can use  

/dev/root / auto defaults     0 0 

in fstab to mount / as readwrite.

recently, I recall this old feature, and tryed to use it, but the system 
failed to boot, waiting for /dev/root that never come up.

how to bring back the good old feature that re-use the root device assigned on 
kernel command line ?