cmake: Use path-corrected version of ZEPHYR_BASE

Instead of accessing the environment variable ZEPHYR_BASE every time we
require accessing the source code root, use an intermediate variable
that has OS path separators correctly set to '/' to avoid issues on
Windows.

Note: This removes the ZEPHYR_SOURCE_DIR CMake variable. External
applications using that will need to change to use the new ZEPHYR_BASE
variable.

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