Configure a basic CMake library

[glebm]

Provides 2 targets: Portduino::Portduino and Porduino::PortduinoMain

Can be used like so:

include(FetchContent)
FetchContent_Declare(Portduino
  GIT_REPOSITORY https://github.com/glebm/framework-portduino.git
  GIT_TAG ...
  EXCLUDE_FROM_ALL)
FetchContent_MakeAvailable(Portduino)

...

target_link_libraries(MyProject PRIVATE Portduino::Portduino)

Refs #33

[CLAassistant]

All committers have signed the CLA.

[mverch67]

The current CMakeLists.txt in master seems not maintained, it's broken. This updated CMakeLists.txt works nicely from CLI and within vscode:

[main] Building folder: /home/manuel/Documents/PlatformIO/Projects/framework-portduino/build 
[build] Starting build
[proc] Executing command: /usr/bin/cmake --build /home/manuel/Documents/PlatformIO/Projects/framework-portduino/build --config Debug --target all -j 18 --
[build] [ 16%] Building CXX object CMakeFiles/Portduino.dir/cores/arduino/api/String.cpp.o
[build] [ 16%] Building CXX object CMakeFiles/Portduino.dir/cores/arduino/api/Common.cpp.o
[build] [ 16%] Building CXX object CMakeFiles/Portduino.dir/cores/arduino/api/IPAddress.cpp.o
[build] [ 22%] Building CXX object CMakeFiles/Portduino.dir/cores/arduino/api/Print.cpp.o
[build] [ 27%] Building C object CMakeFiles/Portduino.dir/cores/portduino/dtostrf.c.o
[build] [ 38%] Building CXX object CMakeFiles/Portduino.dir/cores/arduino/api/Stream.cpp.o
[build] [ 38%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/itoa.cpp.o
[build] [ 66%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/linux/millis.cpp.o
[build] [ 66%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/simulated/SimCommon.cpp.o
[build] [ 66%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/simulated/SimHardwareSPI.cpp.o
[build] [ 66%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/simulated/SimHardwareI2C.cpp.o
[build] [ 66%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/Utility.cpp.o
[build] [ 72%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/linux/LinuxSerial.cpp.o
[build] [ 77%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/linux/LinuxHardwareI2C.cpp.o
[build] [ 83%] Building CXX object CMakeFiles/Portduino.dir/cores/portduino/linux/LinuxCommon.cpp.o
[build] [ 88%] Linking CXX static library libPortduino.a
[build] [ 88%] Built target Portduino
[build] [ 94%] Building CXX object CMakeFiles/PortduinoMain.dir/cores/portduino/main.cpp.o
[build] [100%] Linking CXX static library libPortduinoMain.a
[build] [100%] Built target PortduinoMain
[driver] Build completed: 00:00:01.691
[build] Build finished with exit code 0 

It's nice to have a quick way to check if all files are compile clean and the library can be build whenever a change is done.

HOWEVER:
The list of files seems incomplete in this makefile. Some files are missing (logging., FS/, linux/gpio/), some are commented out (PluggableUSB.). Would be good to sort the files alphabetically then they compile in the same order as platformio does it and it's easier to search for and add missing files.