Kconfiglib: Fix $srctree behavior for the top-level Kconfig file

Due a design goof, the top-level Kconfig file passed to
Kconfig.__init__() wasn't looked up relative to $srctree. This broke a
planned fix for an issue with the CI check for references to undefined
Kconfig files in external projects.

Update Kconfiglib to upstream revision 953cc12464dae to fix it.

Also update doc/CMakeLists.txt to pass 'Kconfig' instead of '../Kconfig'
to genrest.py. $srctree is already set to $ZEPHYR_BASE.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 5711a64..df95e77 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -104,7 +104,7 @@
   ENV_VAR_ARCH=*
   KERNELVERSION=${PROJECT_VERSION}
   SRCARCH=x86
-  ${PYTHON_EXECUTABLE} scripts/genrest.py ../Kconfig ${RST_OUT}/doc/reference/kconfig/
+  ${PYTHON_EXECUTABLE} scripts/genrest.py Kconfig ${RST_OUT}/doc/reference/kconfig/
   WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}
 )