[nrf temphack] cmake: carry forward dts.cmake patches from a mergeup

Mergeup commit 1378a1c made changes
to cmake/dts.cmake which I'm carrying forward in a separate patch for
now.

Need more clarity from Robert or Sebastian on what "ought" to happen to
this patch:

     1. squash into multi-image?
     2. submit upstream?

Signed-off-by: Marti Bolivar <[email protected]>
Signed-off-by: Robert Lubos <[email protected]>
Signed-off-by: Sebastian Bøe <[email protected]>
(cherry picked from commit b5e975a)
Signed-off-by: Martí Bolívar <[email protected]>

Author: mbolivar-nordic

cmake/dts.cmake

@@ -28,6 +28,9 @@ list(APPEND
   ${BOARD_DIR}
   ${ZEPHYR_BASE}
   )
+list(REMOVE_DUPLICATES
+  DTS_ROOT
+  )
 
 set(dts_files
   ${DTS_SOURCE}
@@ -74,6 +77,7 @@ if(SUPPORTS_DTS)
     math(EXPR i "${i}+1")
   endforeach()
 
+  unset(DTS_ROOT_SYSTEM_INCLUDE_DIRS)
   foreach(dts_root ${DTS_ROOT})
     foreach(dts_root_path
         include
@@ -91,6 +95,7 @@ if(SUPPORTS_DTS)
     endforeach()
   endforeach()
 
+  unset(DTS_ROOT_BINDINGS)
   foreach(dts_root ${DTS_ROOT})
     set(full_path ${dts_root}/dts/bindings)
     if(EXISTS ${full_path})