from the man page mdmfs(8)
....
The same file system created as an entry in /etc/fstab:
md /tmp mfs rw,-s32m 2 0
...
shouldn't it read
The same file system created as an entry in /etc/fstab:
md /tmp mfs rw,-s32m 0 0
??
the boot process stalls when md /tmp mfs rw,-s32m 2 0 is in fstab