cmake: kconfig: pass GENERATED_DTS_BOARD_CONF on to kconfig

Make kconfigfunctions.py agnostic to where GENERATED_DTS_BOARD_CONF is
located.

We don't want to encode the path of GENERATED_DTS_BOARD_CONF into more
places than necessary. Also, re-using GENERATED_DTS_BOARD_CONF makes
it easier to change where generated_dts_board.conf is located.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
index 6a29112..1254b0c 100644
--- a/doc/CMakeLists.txt
+++ b/doc/CMakeLists.txt
@@ -147,6 +147,7 @@
   SOC_DIR=soc/
   SRCARCH=x86
   PROJECT_BINARY_DIR=${CMAKE_CURRENT_BINARY_DIR}
+  GENERATED_DTS_BOARD_CONF=not_applicable
   KCONFIG_TURBO_MODE=${KCONFIG_TURBO_MODE}
   KCONFIG_DOC_MODE=1
   ${PYTHON_EXECUTABLE} scripts/genrest.py Kconfig ${RST_OUT}/doc/reference/kconfig/