cmake: Configure custom linker script Kconfig before usage

Allows having CMake variables inside of this Kconfig to e.g. use a
value that can specify a board directory or path relative to a
CMake file location variable

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

CMakeLists.txt

@@ -1109,10 +1109,10 @@ set_ifndef(  TOPT "${COMPILER_TOPT}")
 set_ifndef(  TOPT -Wl,-T) # Use this if the compiler driver doesn't set a value
 
 if(CONFIG_HAVE_CUSTOM_LINKER_SCRIPT)
-  set(LINKER_SCRIPT ${APPLICATION_SOURCE_DIR}/${CONFIG_CUSTOM_LINKER_SCRIPT})
+  string(CONFIGURE ${APPLICATION_SOURCE_DIR}/${CONFIG_CUSTOM_LINKER_SCRIPT} LINKER_SCRIPT)
   if(NOT EXISTS ${LINKER_SCRIPT})
-    set(LINKER_SCRIPT ${CONFIG_CUSTOM_LINKER_SCRIPT})
-    assert_exists(CONFIG_CUSTOM_LINKER_SCRIPT)
+    string(CONFIGURE ${CONFIG_CUSTOM_LINKER_SCRIPT} LINKER_SCRIPT)
+    assert_exists(LINKER_SCRIPT)
   endif()
 elseif(DEFINED BOARD_LINKER_SCRIPT)
   set(LINKER_SCRIPT ${BOARD_LINKER_SCRIPT})