cmake: do not double escape double quote

Do not escape single escaped quote '\"'.
A single escape quote in CMake indicates the use of literal '"'.
Escaping those results in '\\"' which is a literal '\' and a quote which
encapsulates a string.
This is a result we do not want.
Therefore, exempt the '\"' sequence from further escaping.

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