Complete base software/firmware environment for further production needs

[forGGe]

A short-term goal for the firmware is to demonstrate a prototype operation. Long term: be extendable enough to cover all (potential) final requirements. Robustness and verbosity (how device advertises its current state) are highest priorities here.

In the scope of this task:

• [x] Pick a framework to start with: mbed, cubemx, etc.
• [x] Pick a ModBus library, suitable for uC applications
• [ ] Add ModBus control over sensor parameters, such as precision, etc.
• [x] Add ModBus reading of sensor data.
• [x] Add debug output to log device states and faults

Optional in this task, but definitely should be implemented after:

• [ ] Add unit-test framework (host) and test placeholders
• [ ] Console commands to control a device
• [ ] POST and basic factory tests

[forGGe]

Framework selected: Zephyr, v1.14.0 ModBus library: self-implemented. Rationale: no useful embedded lib for ASCII slave communication. However, it not so hard to implement it. ModBus controls moved to the next stage, no need to implement it now.

Test framework: must be based on, Zephyr, postponed. Shell/console commands: are part of Zephyr, that convered as well. POST and other things: moved to the next phase.