cmake: Fix path handling for ZEPHYR_BASE

With the addition of a local CMake variable ZEPHYR_BASE that mirrors the
value of the environment variable the documentation build broke on
Windows. This is because CMake was incorrectly interpreting backslashes
present in the Windows path as escaping characters. In order to avoid
this, use CMake's built-in file(TO_CMAKE_PATH) functionality, which
converts the path to a canonical version using forward slashes only.
At the same time rework boilerplate.cmake to use the same mechanism
instead of manually replacing backslashes and removing trailing ones.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>