Zephyr support as a module

[xerpi]

I think it would be nice to have liquid-dsp support as a Zephyr module to facilitate integration into embedded projects using Zephyr’s build system. (There's already CMSIS-DSP support in Zephyr.)
Zephyr’s module system relies on CMake for seamless inclusion of external libraries.

Examples of Zephyr Modules:

• Sample Project:
  • Library: https://github.com/yashi/module-sample
  • Application: https://github.com/yashi/module-app
• libcsp:
  • Library: https://github.com/libcsp/libcsp/tree/develop/contrib/zephyr
  • Application: https://github.com/libcsp/libcsp-zephyr