Q: simulate ok, but no root password prompt show up 
A: 
i think it is because i reboot the system, original swap maybe mix up (buildworld many times, build kernel many times) 
after reboot, i rebuild world once, then do all kind of rebuild kernel

Q: i386/GERERIC compiled, root prompt does NOT show (type in root, but hang there)
A: 
 1 modify GENERIC to SUN, get rid of something then ./nanobsd.sh -w (skip buildworld to save time)
 2 try again (SUN) FAIL too (not buildworld)
 3 with buildworld (FAIL)
 4 rm /usr/embedded/* then build whole thing again (FAIL)
 5 at host root change its password by typing passwd and then ../nanobsd.sh -k -w (go directly into build image FAIL)
 6 single user mode to reset root password (succeed!)

solved: single user mode reset root password (by qemu-system-x86_64 -singlestep)