Build instructions for RNode_Firmware

[dllud]

Could you please add command-line build instructions for RNode_Firmware?
(In the README.md or a separate BUILDING.md)
Adding a dependencies' list would help too.

I sifted through the repo and could only find flashing instructions and vague mentions about using the Arduino IDE.

Being able to build and flash from the command line allows developers to choose their preferred IDE and eases any future automation work (e.g. a CI/CD).

[markqvist]

Yes, that'd be a good idea to have stated explicitly.

On Linux, it should be sufficient to install the base development packages, along with arduino-cli. After this, you can simply use make to prepare, build and flash for all supported targets. For ESP32, for example:

make prep-esp32
make firmware-tbeam_sx1262
make upload-tbeam_sx1262

Or build a full release package for all supported devices:

make release