Do nextboot -D twice during boot. The first time in rc.d/root which ensures that

we can remove the file as early as possible, but shut up nextboot at this moment
if the operation is failed, because /boot is not necessarily a part of /; the
newly added second run is placed in rc.d/mountlate after all filesystems were
mounted.

Discussed at:		-rc@
Suggestions from:	brooks, mtm
MFC after:		1 month

etc/rc.d/root

@@ -34,7 +34,7 @@ root_start()
 	# If we booted a special kernel remove the record
 	# so we will boot the default kernel next time.
 	if [ -x /sbin/nextboot ]; then
-		/sbin/nextboot -D
+		/sbin/nextboot -D > /dev/null 2>&1
 	fi
 }