llext: custom target properties, add_llext_command example

Allow external code to interact with the llext build process by publishing
custom properties on the targets created by add_llext_target(). This is
especially useful to custom commands, since they may need to retrieve
specific details about the targets on which they can operate.

Add an example usage of the new properties and add_llext_command() to
the hello_world test case.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>

cmake/modules/extensions.cmake

@@ -5149,6 +5149,15 @@ endfunction()
 # flags to remove and flags to append is controlled respectively by the
 # LLEXT_REMOVE_FLAGS and LLEXT_APPEND_FLAGS global variables.
 #
+# The following custom properties of <target_name> are defined and can be
+# retrieved using the get_target_property() function:
+#
+# - LibTarget   Target name for the source compilation and/or link step.
+# - LibOutput   The binary file resulting from compilation and/or
+#               linking steps.
+# - PkgInput    The file to be used as input for the packaging step.
+# - PkgOutput   The final .llext file.
+#
 # Example usage:
 #   add_llext_target(hello_world
 #     OUTPUT  ${PROJECT_BINARY_DIR}/hello_world.llext
@@ -5285,12 +5294,18 @@ function(add_llext_target target_name)
     message(FATAL_ERROR "add_llext_target: unsupported architecture")
   endif()
 
-  # Add user-visible target and dependency
+  # Add user-visible target and dependency, and fill in properties
   get_filename_component(output_name ${llext_pkg_output} NAME)
   add_custom_target(${target_name}
     COMMENT "Generating ${output_name}"
     DEPENDS ${llext_pkg_output}
   )
+  set_target_properties(${target_name} PROPERTIES
+    LibTarget ${llext_lib_target}
+    LibOutput ${llext_lib_output}
+    PkgInput  ${llext_pkg_input}
+    PkgOutput ${llext_pkg_output}
+  )
 endfunction()
 
 # Usage:

tests/subsys/llext/simple/CMakeLists.txt

@@ -26,3 +26,14 @@ foreach(ext_name hello_world logging)
   )
   generate_inc_file_for_target(app ${ext_bin} ${ext_inc})
 endforeach()
+
+# Dummy custom processing command to test add_llext_command
+get_target_property(proc_in_file hello_world_ext LibOutput)
+get_target_property(proc_out_file hello_world_ext PkgInput)
+
+add_llext_command(
+  TARGET hello_world_ext
+  POST_BUILD
+  COMMAND echo "dummy patching ${proc_in_file} to create ${proc_out_file}"
+  COMMAND ${CMAKE_COMMAND} -E copy ${proc_in_file} ${proc_out_file}
+)