build: find wl protocols using sysroot and generate them inside build folder

[AndreCostaaa]

Simple enhancements to the wl_protocols generation script:

• Allow defining the output folder and replace the previously used wl_protocols folder to the build folder in cmake
  • I wasn't liking having to manually remove two folders to do a clean build
• Allow easy addition of new protocols as I have a feeling we will needed this in not so distant future
  • To support new we simply need to echo the generated private-code and it will be collected by cmake
• Prefix the protocols folder with $SYSROOT to support cross-compilation by finding protocols in the target system's sysroot rather than the host system

cc @etag4048 @anaGrad