Make Zephyr port work as a Zephyr module

[hasheddan]

There is currently a Zephyr module file in this repository, but it cannot be used because it references cmake-ext and kconfig-ext, despite CMakeLists.txt and Kconfig not being present for WAMR in Zephyr upstream modules. There was discussion about including upstream in https://github.com/zephyrproject-rtos/zephyr/issues/21329, but it appears that the decision for now is for it to live separately. With that being the case, we should make it easy for Zephyr users to pull WAMR in as an external module. This primarily just consists of adding an appropriate CMakeLists.txt and Kconfig in the WAMR repository, then pointing to them via the zephyr/module.yml file.

I'd be happy to make these updates -- @wenyongh what do you think?

[wenyongh]

@hasheddan Yes, that's great! Shall we add the CMakeLists.txt and Kconfig under <wamr_root>/zephry folder? There is a CMakeLists.txt under <wamr_root> already.

[hasheddan]

@wenyongh either there or https://github.com/bytecodealliance/wasm-micro-runtime/tree/main/core/shared/platform/zephyr is great with me -- do you have a preference? I'll get started adding it!

[wenyongh]

@hasheddan had better put them under <wamr_root>/zephry, core/shared/platform/zephyr may be too deep and not easy to maintain.

[hasheddan]

@wenyongh will do, thanks!