楼主，这个问题我也遇到了，现在解决了。步骤如下：
1、重新编译busybox。
输入命令
#make menuconfig
在菜单的 Login/Password Management Utilities ---> 下按照如下选择：
[*] Support for shadow passwords
[*] Use internal password and group functions rather than system functions
[*] Use internal shadow password functions
[*] Use internal crypt functions
[*] Enable SHA256/512 crypt functions
[*] addgroup
[*] Enable long options
[*] Support for adding users to groups
[*] delgroup
[*] Support for removing users from groups
[ ] Enable sanity check on user/group names in adduser and addgroup
[*] adduser
[ ] Enable long options
(100) First valid system uid or gid for adduser and addgroup
(999) Last valid system uid or gid for adduser and addgroup
[*] deluser
[*] getty
[*] login
[ ] Support for PAM (Pluggable Authentication Modules)
[*] Support for login scripts
[*] Support for /etc/nologin
[*] Support for /etc/securetty
[*] passwd
[*] Check new passwords for weakness
[*] cryptpw
[*] chpasswd
[*] su
[*] Enable su to write to syslog
[*] Enable su to check user's shell to be listed in /etc/shells
[*] sulogin
[*] vlock
2、拷贝三个文件 passwd shadow inittab 到 /etc 下
passwd 与 shadow 文件就不说了， inittab 文件内容如下
# /etc/inittab
# The default runlevel is defined here
#:3:initdefault:
# System initialization.
::sysinit:/etc/init.d/rcS
#1:2345:respawn:/sbin/getty 115200 tty1
#2:2345:respawn:/sbin/getty 115200 tty2
#3:2345:respawn:/sbin/getty 115200 tty3
#4:2345:respawn:/sbin/getty 115200 tty4
#5:2345:respawn:/sbin/getty 115200 tty5
#6:2345:respawn:/sbin/getty 115200 tty6
# Put a getty on the serial line (for a terminal)
# uncomment this line if your using a serial console
#:2345:respawn:/sbin/getty -L s3c2410_serial0 115200 vt100
:2345:respawn:/sbin/getty -L ttySAC0 115200 vt100
#::askfirst:-/bin/sh
:12345:ctrlaltdel:/sbin/reboot
