zephyr: CMake portability fixes - #6611

@dpgeorge This PR is based upon the comments I made in #6542 (comment)

It's not directly mergeable, since there were differences between the two branches I've not yet fully reconciled. It looks like some changes were merged which obsoletes some of what I was doing. However, I hope some of the general ideas here are useful:

• The main part is the last bit which adds quote and dequote scripts to replace the use of sed.
• shell commands like echo and sed are replaced with CMake-native replacements. This provides for portability beyond UNIX e.g. to Windows
• some configuration currently done at build-time can be done at configure-time, e.g. version header generation. There's no real need for this to be a build rule. This means it won't regenerate the header upon every invocation of the build. I added a bit of logic to make it automatically regenerate whenever the git HEAD changes, which means it will still happen, but less frequently. That could be extended to also include other sources of version information, e.g. documentation.
• looking more closely, I do wonder if all of the qstr processing couldn't be done at configure time and regenerated on demand if needed