Allow setting NANOPB_SRC_ROOT_FOLDER

[axel-h]

• in CMake, INTERNAL has the same effect as FORCE, so passing any -DNANOPB_SRC_ROOT_FOLDER=... to CMake on the command line will be overwritten and there is no way to customize the path then.
• the second commit removes the trailing slash of the variable, which seem more a cosmetic thing. Having a "//" on some paths in the variables that are created later seems not to cause any practical issues.

[Indanz]

Out of curiosity, why are you using a different NanoPB and why can't you put it in tools/nanopb instead? Or symlink tools/nanopb to wherever your nanopb is?

[axel-h]

I am using the same nanoPB version, but just a different directory layout. Symlinking is not really an option, as this would be the only thing that needs the project workspace and tools folder then.

I did not get things to work with just adding nanoPB to the CMAKE_MODULE_PATH. Making this work could be another mire generic way, but it may need more changes.

[Indanz]

The directory layout seems quite hard-coded, how do you manage the other tools paths? Is it worth trying to re-use settings.cmake? It doesn't seem to be doing that much useful work.

[axel-h]

Providing -DCMAKE_MODULE_PATH=... works well enough for all other modules.

[axel-h]

Concerning your question about using settings.cmake or not, that is where #40 is aiming for to allow full customization. But as stated there, settings.cmake contains some useful parts, so using it has some benefits.

[axel-h]

Since sel4bech basically uses the same settings.cmake, but does not need the patch, I have been digging a bit more into this. It seems sel4testis running in to the issues described at https://stackoverflow.com/questions/75619926/does-cmake-set-internal-apply-force because CMake runs settings.cmake twice here (at least when targeting QEMU, the logs show this). The second runs finally makes INTERNAL act in FORCE-mode and overwrite the command line setting. In sel4bench it runs only once, so the command line value is used.
I still think we should not use INTERNAL here (and also apply this patch in sel4bench). There is a message we print in https://github.com/seL4/seL4_tools/blob/8c660fd6d8ba60aa4c76d5c9586e3892502d8e0e/cmake-tool/helpers/nanopb.cmake#L22 that also implies this values is supposed to be be something one can set on the command line.