feature request: allow use of etl::mutex on non-OS targets

[KammutierSpule]

I would like to include etl/mutex.h and etl:mutex but to be used on non-OS (or custom) targets It may be allowed to include a user include file or some other way that can result on a "compile to nothing" or user implementation.

[enbyted]

@KammutierSpule

Having an implementation of mutex that does nothing in the library could be quite dangerous as it would open the possibility of compiling code that expects a working mutex with a null implementation without any warning.

What I do is dependent on the usecase, but typically I:

• In case of general-use libraries: Make my classes generic over the mutex type (etl::lock_guard is provided even when etl::mutex is not)
• In case of portable executables: Provide a custom typedef for the mutex type and use that typedef in the codebase. The typedef is set based on the ETL_HAS_MUTEX macro or by some config options, depending on context
• In case of non-portable executables or libraries: Just use etl::mutex, worst case scenario when having to port to a non-supported platform I will have to provide some polyfill or refactor the existing implementation.

Hopefully that's helpful to you.

// I'm not not related to ETL, just random user