You need to use the iproute2 commands and ifconfig commands are deprecated.
Try this:
ip addr flush dev eth0 # remove running ip config
ip addr add xxx.xxx.xxx.xxx/xx dev eth0 # set ip for dev eth0
ip addr show
ip route add default via xxx.xxx.xxx.xxx dev eth0 # set default gateway
It should work but i don't know how to make this survive a reboot, didn't have time to play with config files yet.