cmake: escape json input string

Escape the json input string given to `to_yaml()` function and the
content given to `yaml_set()` function.

This ensures that a string like `foo\bar` becomes `foo\\bar` during
internal CMake json processing and when written to the file it becomes
`foo\bar`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>