tools/serial: add support for socat as riot terminal

[aabadie]

Contribution description

This PR provides support for socat as an alternative RIOT terminal tool. It was inspired by #11003.

The good thing with socat is that it allows redirecting the serial port to anything: the current shell, a TCP server, etc. The default behaviour is to redirect to the current shell like other tools. But it's possible to provide the SOCAT_OUTPUT variable with custom output parameters.

Testing procedure

• Just build and flash any application for a given board with a shell
• Open the terminal using socat:

$ RIOT_TERMINAL=socat make BOARD=saml11-xpro -C examples/default/ flash term
> help

• Try the redirection to a different output, here a local TCP server:

$ SOCAT_OUTPUT=TCP4-LISTEN:20000,reuseaddr RIOT_TERMINAL=socat make BOARD=saml11-xpro -C examples/default/ flash term

• You can access the shell using netcat from another terminal (this is exactly what is done in IoT-LAB for instance):

$ nc localhost 20000
> help

Issues/PRs references

Inspired by #11003

[kaspar030]

Works like a charm. ACK.

[aabadie]

Thanks for merging @kaspar030!
I wasn't expecting this to be merged that fast though :)

[kaspar030]

I wasn't expecting this to be merged that fast though :)

Should we revert? 😉