Add weak attribute to mcu_api.c

[disk91]

As the previous commit, it allows to override the api function and not touch the sigfox submodule

[Ludovic-Lesur]

Thank you @disk91 for this improvement proposal. The __attribute__((weak)) keywork will be added on all functions template on the next versions (library and other repositories which have templates as well).

Meanwhile, most of IDEs feature an exclude from build option, telling the compiler to ignore specific files or folders. If you apply this to the src/manuf folder of the sigfox-ep-lib repository, you can implement your own mcu_api.c and rf_api.c files while keeping the submodule clean.