Add SLIP support

[ssshake]

It's possible to allow internet access via slip but currently that would require flashing different firmware depending on which mode you want the modem to run in. A goal would be to consolidate them into one.

[jhhoward]

Perhaps this could be a different firmware, but I imagine these features would work well together:

• ESP runs as a SLIP server to forward IP traffic to the connected machine
• ESP runs a DHCP service to allocate an IP address to the connected machine
• ESP hosts a telnet server for a simple interface for configuring the network, e.g. viewing different wireless access points, setting the passwords etc
• ESP could optionally run a web server for configuring through a web interface
• ESP could run an HTTPS -> HTTP proxy to allow the connected machine to connect to modern websites easily

[ssshake]

These are great ideas! I have a lot to learn before I could implement them.

For the configuration side of things, I have a good deal of experience hosting web servers on these ESP's. A web page could be used to set all of the config. You can even do out-of-box wifi set up quite easily by connecting to it from your phone. I've done this on other projects and that would be great for this one.

I'm adding some of the links you sent me privately here for reference later:

https://mcmackins.org/stories/dos-slip.html
https://github.com/martin-ger/esp_slip_router