When you boot "-s" you end up in single-user mode. It is a root shell, with no environment variables set other than $TERM. Only the root partition "/" is mounted, and it is mounted read-only.
ifconfig is normally installed in /sbin, which is in the root partition. You should be able to use it. However, /var is usually a separate filesystem, and is not mounted. And your dmesg won't be in /var/run/dmesg.boot, as rc(8) is not run in single user mode. If you want to see your dmesg, issue the dmesg(8) command.
To mount all of your automatically mounted filesystems from fstab(5), issue:
# mount -a
|