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 953cc124 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>