Unified configuration

[mcspr]

@lbdroid user no longer exists and #1839 was deleted (wtf?)

Given that it is now becoming possible to secure MQTT; #1829

I would like to be able to pre-configure all user settings in the firmware prior to deploying to a device. My goal is to be able to write the firmware onto a device, plug it in, and not have to make any configurations to the device at all.

In achieving this, it would be possible to entirely disable both WEB and TELNET interfaces. This provides the double benefit of reducing the size of the firmware, as well as minimizing the attack surface.

Some configurations are already possible via custom.h, but nowhere near all and not enough to avoid having to make manual configurations (or "restore" a json encoded backup).

But there is also the question of having two ways of setting everything and all of the excess code it results in. Simpler and more easily maintained approach would be to eliminate the majority of the defines-based configurations and replace it with a list of default key-value pairs to be loaded on first boot.

Questions are:

• is it possible to expose key-value configuration at build-time
• is configuration through header defines detrimental for an easy user configuration
  (and can we live without -D... in build_flags / src_flags for env)
• can this all be somehow compatible with Arduino IDE / arduino-cli (arduino-builder)

[lbdroid]

Yeah, GitHub sucks.