Replace Mongoose with Civetweb

RTEMS has an old version of Mongoose using a permissive license before it was relicensed to GPL. There was a fork at the point of it being relicensed and it is now called Civetweb (https://github.com/civetweb/civetweb).

This project would remove mongoose from RTEMS and ideally replace it with an RSB built version of Civetweb. This would enable it to be used on top of any TCP/IP stack RTEMS supports.

One challenge will be figuring out how to deal with all the options Civetweb has, making these accessible to the user building Civetweb via RSB, and documenting them.