cmake: hide internal targets

Use imported library, which does not create additional rules, but still
allows attaching arbitraty properties instead of custom targets. This
allows the targets to not appear in the target list of IDEs such as
CLion.

tools/cmake/build.cmake

@@ -128,7 +128,7 @@ endfunction()
 #
 function(__build_init idf_path)
     # Create the build target, to which the ESP-IDF build properties, dependencies are attached to
-    add_custom_target(__idf_build_target)
+    add_library(__idf_build_target STATIC IMPORTED)
 
     set_default(python "python")
 

tools/cmake/component.cmake

@@ -173,7 +173,7 @@ function(__component_add component_dir prefix)
     # 'override' components added earlier.
     if(NOT component_target IN_LIST component_targets)
         if(NOT TARGET ${component_target})
-            add_custom_target(${component_target} EXCLUDE_FROM_ALL)
+            add_library(${component_target} STATIC IMPORTED)
         endif()
         idf_build_set_property(__COMPONENT_TARGETS ${component_target} APPEND)
     endif()