CMakeLists.txt: let picowota_build_combined add files to clean target

We observed strange issues that (among other things) revealed that make clean
did not really make things as clean as it could. This is an attempt to clean up
various files generated.

CMakeLists.txt

 	get_target_property(PICOWOTA_SRC_DIR picowota SOURCE_DIR)
 	get_target_property(PICOWOTA_BIN_DIR picowota BINARY_DIR)
 
+	get_directory_property(initial_clean ADDITIONAL_MAKE_CLEAN_FILES)
+	list(APPEND clean_files ${initial_clean})
+	list(APPEND clean_files ${APP_BIN})
+	list(APPEND clean_files ${APP_HDR_BIN})
+
 	# The app must be built with the correct linker script (and a .bin)
 	picowota_build_standalone(${NAME})
 
 			--update-section .app_hdr=${APP_HDR_BIN}
 			--update-section .app_bin=${APP_BIN} ${PICOWOTA_BIN_DIR}/picowota.elf ${COMBINED}.elf
 	)
+	list(APPEND clean_files ${COMBINED}.bin)
+	list(APPEND clean_files ${COMBINED}.elf)
 
 	add_custom_command(TARGET ${COMBINED} POST_BUILD
 		COMMAND ${CMAKE_OBJCOPY} -Obinary ${COMBINED}.elf ${COMBINED}.bin
 	if (ELF2UF2_FOUND)
 		add_custom_command(TARGET ${COMBINED} POST_BUILD
 			COMMAND ELF2UF2 ${COMBINED}.elf ${COMBINED}.uf2
-	)
+		)
+		list(APPEND clean_files ${COMBINED}.uf2)
 	endif()
+
+	set_directory_properties(PROPERTIES ADDITIONAL_MAKE_CLEAN_FILES "${clean_files}")
 endfunction()