Can't boot Marvel Sheevaplug from USB

[Jack Avenger]

Which options should be included in kernel configuration to boot 8- 
CURRENT on SHEEVAPLUG?

I tried options ROOTDEVNAME="ufs:/dev/da0s1a", but no success:

usbus0: 480Mbps High Speed USB v2.0
Root mount waiting for: usbus0
ugen0.1: <Marvell> at usbus0
uhub0: <Marvell EHCI root HUB, class 9/0, rev 2.00/1.00, addr 1> on  
usbus0
uhub0: 1 port with 1 removable, self powered
Root mount waiting for: usbus0
ugen0.2: <Verbatim> at usbus0
umass0: <Verbatim STORE N GO, class 0/0, rev 2.00/1.10, addr 2> on  
usbus0
umass0:  SCSI over Bulk-Only; quirks = 0x0000
Root mount waiting for: usbus0
umass0:0:0:-1: Attached to scbus0
Trying to mount root from ufs:/dev/da0a
ROOT MOUNT ERROR:
If you have invalid mount options, reboot, and first try the following  
from
the loader prompt:

      set vfs.root.mountfrom.options=rw

and then remove invalid mount options from /etc/fstab.

Loader variables:
vfs.root.mountfrom=
vfs.root.mountfrom.options=

Manual root filesystem specification:
   <fstype>:<device>  Mount <device> using filesystem <fstype>
                        eg. ufs:/dev/da0s1a
                        eg. cd9660:/dev/acd0
                        This is equivalent to: mount -t cd9660 /dev/ 
acd0 /

   ?                  List valid disk boot devices
   <empty line>       Abort manual input

mountroot>

it seems like there is an error accessing USB Flash, but it work when  
boot from NFS is compiled into kernel.