[oe] mdev.sh: permit serial console login and permit correct runlevel

[Denis 'Gnutoo' Carikli]

Hi, This patch is permit the login via serial console,and to go to the correct runlevel(2345),
without this patch init is blocked at the last script of runlevelS and we can never login via serial console
(the patch created /dev/null and /dev/console)
I send it for review because it touch a core part and that I'm not shure how the others distro handle that.
Here I used:
*wrt54.conf
*micro-base-image
*micro-uclibc

Denis