Add option to build LoRaMac sources as a static library

[dmeehan1968]

Intention of this PR is to allow developers to build the LoRaMac sources as a static library for inclusion in their own projects.

A separate directory is added to the project called as-lib which contains a set of CMakeLists.txt files that shadow the directory structure under src. A backwards compatible change is made to the project root CMakeLists.txt which, when provided with the option LORAMAC_AS_LIB, will direct CMake to reference the as-lib directory instead of src. The option default is OFF so that the past build behaviour is preserved as the default.

A README.md is provided in the as-lib directory with an outline of options/settings that can be applied to the build. A standard LORAMAC_ prefix is applied to all options to aid interoperatibility with other project sources.

This is a rough draft and is currently working within my own project. I will add usage instructions to the README shortly.

I'm currently loading the repo into the project via CMake's FetchContent, as this allows the inclusion of the project root via add_subdirectory whilst specifying other options, such as building regional specific variants (my own projects tend to have different builds for each desired LoRaWAN region, rather than a single multi-region build (although this is also intended to be supported by this facility).

Feedback, enhancements and criticism welcome.

[MedadRufus]

Would be useful.

[mluis1]

Thanks for the proposition.
We will consider it for a future major release.

[dmeehan1968]

There is an argument with this that its used to rework the examples (using the parallel directory structure to leave the legacty as-is) so that they are built using the static library targets. In time, this can then be worked back into the src tree.

I don't see that as being a change of interface (respect your choice as to when to integrate of course).

I can't really do the range of examples due to lack of hardware/toolchain. In fact I only have our custom boards which are only MCU/radio similar to the B-L027-LRWAN-1 example, but it's at least the same toolchain. I could do an example based off that if you're interested. Do you think that should be another PR, or add to this?

[dmeehan1968]

Rebased onto current master, v4.7.0 (plus comment fix)

[danak6jq]

Watching this closely.