Replace RCP vendor extension find package with include command

This change updates the mechanism used by vendor.cmake to include a cmake file
to define the required vendor targets used by the rcp vendor interface library instead of
the find_package command that was previously used.

The find_package command would allow for a package to be searched for and a target to be linked
unconditionally if defined by the CMake project. This would lead to problems if the target was already
defined, perhaps as a subdirectory of a super project, and the cache variable is still set for the
openthread build. Additionally, this change also brings the vendor extension of the posix library
more in line with the cli vendor extension, which was created after find_package was used in
vendor.cmake.

src/posix/platform/FindExampleVendorDeps.cmake

@@ -36,9 +36,7 @@ files used by a vendor implementation in the posix library.
 
 The name of this file and the name of the targets it defines are
 conventionally related. For the purpose of this reference, targets
-will be based off of the identifier "ExampleRcpVendorDeps". Derived
-references should be based off of the value of the cache variable,
-"OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE".
+will be based off of the identifier "ExampleRcpVendorDeps".
 
 For more information about package resolution using CMake find modules,
 reference the cmake-developer documentation.

src/posix/platform/vendor.cmake

@@ -30,7 +30,10 @@ set(OT_POSIX_CONFIG_RCP_VENDOR_INTERFACE "vendor_interface_example.cpp"
     CACHE STRING "vendor interface implementation")
 
 set(OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE "" CACHE STRING
-    "name of optional external package to link to rcp vendor implementation")
+    "path to CMake file to define and link posix vendor extension")
+
+set(OT_POSIX_RCP_VENDOR_TARGET "" CACHE STRING 
+    "name of vendor extension CMake target to link with posix library")
 
 if(OT_POSIX_CONFIG_RCP_BUS STREQUAL "VENDOR")
     add_library(rcp-vendor-intf ${OT_POSIX_CONFIG_RCP_VENDOR_INTERFACE})
@@ -50,11 +53,9 @@ if(OT_POSIX_CONFIG_RCP_BUS STREQUAL "VENDOR")
     target_link_libraries(openthread-posix PUBLIC rcp-vendor-intf)
 
     if (OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE)
-        set(DEPS_TARGET ${OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE}::${OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE})
-        find_package(${OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE})
-
-        if(${OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE}_FOUND)
-            target_link_libraries(rcp-vendor-intf PUBLIC ${DEPS_TARGET})
+        include(${OT_POSIX_CONFIG_RCP_VENDOR_DEPS_PACKAGE})
+        if (OT_POSIX_CONFIG_RCP_VENDOR_TARGET)
+            target_link_libraries(rcp-vendor-intf PRIVATE ${OT_POSIX_RCP_VENDOR_TARGET})
         endif()
     endif()
 endif()

src/posix/platform/vendor_extension_example.cmake

@@ -0,0 +1,57 @@
+#
+#  Copyright (c) 2023, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+# This file provides an example on how to implement a RCP vendor extension.
+
+# Set name of the cmake target to link with rcp-vendor-intf
+set(OT_POSIX_RCP_VENDOR_TARGET "vendor-lib")
+
+# Create library target using source file "vendor_source.cpp"
+add_library(${OT_POSIX_RCP_VENDOR_TARGET} vendor_source.cpp)
+
+# Include header files located at "${VENDOR_INCLUDE_PATH}"
+target_include_directories(${OT_POSIX_RCP_VENDOR_TARGET}
+    PRIVATE
+        ${VENDOR_INCLUDE_PATH}
+)
+
+set(EXAMPLE_DEPS_ENABLE OFF CACHE BOOL "Include example deps library if enabled")
+
+# Search for and include an optional library using the module file
+# "FindExampleRcpDeps.cmake" at path "${VENDOR_MODULE_PATH}"
+if (EXAMPLE_DEPS_ENABLE)
+    list(APPEND CMAKE_MODULE_PATH ${VENDOR_MODULE_PATH})
+    find_package(ExampleRcpVendorDeps)
+
+    if(ExampleRcpVendorDeps_FOUND)
+        target_link_libraries(${OT_POSIX_RCP_VENDOR_TARGET} 
+            PRIVATE
+            ${ExampleRcpVendorDeps::ExampleRcpVendorDeps}
+        )
+    endif()
+endif()