Network booting with PXE

[User Mike]

I've succesfully put up 4 diskless machines to boot up 4.9-release thru
PXE. They all work almost flawlessly. But when I connected an old 1GB hard
drive into one of them for swap, as NFS swap is a bit slow, it hangs for
some reason at trying to mount the hard drive as / at boot up.

Dmesg reports on all diskless machines:
Adjusted interface xl0
Shutdown interface faith0
Mounting root from ufs:/dev/ad0s1a
no such device 'ad'
setrootbyname failed
ffs_mountroot: can't find rootvp
Root mount failed: 6
Mounting root from nfs:
NFS ROOT: 10.0.10.1:/var/diskless

But on the machine I added disk for swap it never gets past "no such
device 'a0'" because it exists and calls reset of ata0 and something and
hangs there until reset button is pressed.

I'd like to know if there is a way to not try to mount root on local disks
and only NFS. Should this be done in kernel or loader ...? Since the
machine simply sits there I'd appreciate any suggestions.