doc: genrest: Use , instead of : as the separator in --modules

Using ':' as the separator in --modules breaks module specifications
like

    nrfxlib:nrfxlib:C:/Users/Carles/src/ncs/nrfxlib

The ':' in 'C:' gets seen as a separator.

Use ',' instead, which is unlikely to appear in paths (at least in
--modules). Treat a doubled ',,' as a literal ','.

Another option would be ';', but it clashes with how CMake represents
lists, which is awkward.

Also make quoting of arguments with spaces more robust by passing
VERBATIM to add_custom_target().

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 69f94ae..a077590 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -194,7 +194,7 @@
   kconfig
   COMMAND ${CMAKE_COMMAND} -E make_directory ${RST_OUT}/doc/reference/kconfig
   COMMAND ${CMAKE_COMMAND} -E env
-  PYTHONPATH="${ZEPHYR_BASE}/scripts/kconfig${SEP}$ENV{PYTHONPATH}"
+  PYTHONPATH=${ZEPHYR_BASE}/scripts/kconfig${SEP}$ENV{PYTHONPATH}
   ZEPHYR_BASE=${ZEPHYR_BASE}
   srctree=${ZEPHYR_BASE}
   BOARD_DIR=boards/*/*
@@ -207,13 +207,14 @@
   KCONFIG_DOC_MODE=1
     ${PYTHON_EXECUTABLE} scripts/genrest.py ${RST_OUT}/doc/reference/kconfig/
       --keep-module-paths
-      --modules Architecture:arch:${ZEPHYR_BASE}/arch
-                Driver:drivers:${ZEPHYR_BASE}/drivers
-                Kernel:kernel:${ZEPHYR_BASE}/kernel
-                Library:lib:${ZEPHYR_BASE}/lib
-                Subsystem:subsys:${ZEPHYR_BASE}/subsys
-                'External Module':modules:${ZEPHYR_BASE}/modules
+      --modules Architecture,arch,${ZEPHYR_BASE}/arch
+                Driver,drivers,${ZEPHYR_BASE}/drivers
+                Kernel,kernel,${ZEPHYR_BASE}/kernel
+                Library,lib,${ZEPHYR_BASE}/lib
+                Subsystem,subsys,${ZEPHYR_BASE}/subsys
+                "External Module,modules,${ZEPHYR_BASE}/modules"
 
+  VERBATIM
   WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}
   COMMENT "Running genrest.py Kconfig ${RST_OUT}/doc/reference/kconfig/"
 )