Fix for AP and STA modes simultaneously.

[mmmunir966]

I fixed following:

1. AP and STA modes both can work simultaneously.
2. Added gateway address for DHCP server.

[drensber]

Thanks for these. I'll try it out when I have a chance.

One thing I think should be changed, though: I think that the gateway address should both configurable and optional. Rather than just assuming that it will be the same as the address, here should be a CONFIG_ setting representing the gateway address, and if it is not set at all, then there should be no gateway set.