Ability to create out-of-tree MEI clusters

Matter allows for MEI clusters. Currently, there is an example of such a cluster in the tree:  sample-mei-server. If device vendors want to create their own MEI cluster, it currently needs to be included in tree, because app-common would not include information of the out-of-tree MEI cluster.

This commits:
* adds an example for an external MEI cluster
* adds an mei-template.json that creates app-common out-of-tree
* adds build system modifications so that app-common which includes the external MEI cluster is being used when EXTERNAL_APP_COMMON is given to chip_configure_data_model().

config/common/cmake/chip_gn.cmake

@@ -164,7 +164,7 @@ macro(matter_build target)
         ${CHIP_ROOT}/third_party/nlassert/repo/include
         ${CHIP_ROOT}/third_party/nlio/repo/include
         ${CHIP_ROOT}/third_party/nlfaultinjection/include
-        ${CHIP_ROOT}/zzz_generated/app-common
+        # ${CHIP_ROOT}/zzz_generated/app-common
         ${CMAKE_CURRENT_BINARY_DIR}/gen/include
     )
 

config/common/cmake/chip_gn_args.cmake

@@ -161,6 +161,7 @@ endmacro()
 #   DEVICE_INFO_EXAMPLE_PROVIDER Add example device info provider support
 #   PROJECT_CONFIG  Path to the project-specific configuration file
 #   PROJECT_CONFIG_INC_DIR  Extra include dirs for project configurations
+#   PROJECT_APP_COMMON_INC_DIR Extra include dir for custom app-common
 #
 macro(matter_common_gn_args)
     set(options)
@@ -171,6 +172,7 @@ macro(matter_common_gn_args)
         LIB_PW_RPC
         DEVICE_INFO_EXAMPLE_PROVIDER
         PROJECT_CONFIG
+        PROJECT_APP_COMMON_INC_DIR
     )
     set(multiValueArgs
         PROJECT_CONFIG_INC_DIR
@@ -215,6 +217,14 @@ macro(matter_common_gn_args)
     if (ARG_PROJECT_CONFIG_INC_DIR)
         matter_add_gn_arg_list("chip_project_config_include_dirs"             ${ARG_PROJECT_CONFIG_INC_DIR})
     endif()
+    if (ARG_PROJECT_APP_COMMON_INC_DIR)
+        get_filename_component(PROJECT_APP_COMMON_INC_DIR
+            ${ARG_PROJECT_APP_COMMON_INC_DIR}
+            REALPATH
+            BASE_DIR ${CMAKE_SOURCE_DIR}
+        )
+        matter_add_gn_arg_string("chip_project_app_common_include_dir"         "${PROJECT_APP_COMMON_INC_DIR}")
+    endif()
 endmacro()
 
 # Generate the temporary GN arguments file from the settings

config/nrfconnect/chip-module/CMakeLists.txt

@@ -115,6 +115,7 @@ matter_common_gn_args(
     LIB_SHELL       CONFIG_CHIP_LIB_SHELL
     LIB_TESTS       CONFIG_CHIP_BUILD_TESTS
     PROJECT_CONFIG  ${CONFIG_CHIP_PROJECT_CONFIG}
+    PROJECT_APP_COMMON_INC_DIR  ${CONFIG_CHIP_PROJECT_APP_COMMON_INCLUDE_DIRS}
     DEVICE_INFO_EXAMPLE_PROVIDER CONFIG_CHIP_EXAMPLE_DEVICE_INFO_PROVIDER
 )
 matter_add_gn_arg_string("zephyr_ar"                              ${CMAKE_AR})

config/zephyr/Kconfig

@@ -316,6 +316,11 @@ config CHIP_PROJECT_CONFIG
 	  configuration file that can be used to override any of Matter's default or
 	  platform-specific settings.
 
+config  CHIP_PROJECT_APP_COMMON_INCLUDE_DIRS
+	string "Project specific app-common directory for MEI clusters"
+	help
+	  Provides a directory to the app-common/ folder.
+
 # Thread network features
 
 config CHIP_ENABLE_DNSSD_SRP

examples/external-mei-app/external-mei-common/BUILD.gn

@@ -0,0 +1,42 @@
+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/chip.gni")
+import("${chip_root}/src/app/chip_data_model.gni")
+
+config("config") {
+  include_dirs = [ "include" ]
+}
+
+chip_data_model("lighting-common") {
+  zap_file = "lighting-app.zap"
+  is_server = true
+}
+
+source_set("color-format") {
+  public_configs = [ ":config" ]
+  sources = [
+    "include/ColorFormat.h",
+    "src/ColorFormat.cpp",
+  ]
+}
+
+source_set("lighting-manager") {
+  deps = [ "${chip_root}/src/lib" ]
+  public_configs = [ ":config" ]
+  sources = [
+    "include/LightingManager.h",
+    "src/LightingManager.cpp",
+  ]
+}