stm32: common: Fix '_stm32_generate_file'

Fixed bug that occurs when target's property 'RUNTIME_OUTPUT_DIRECTORY'
is not defined.

cmake/stm32/common.cmake

@@ -62,9 +62,15 @@ endfunction()
 
 function(_stm32_generate_file TARGET OUTPUT_EXTENSION OBJCOPY_BFD_OUTPUT)
     get_target_property(TARGET_OUTPUT_NAME ${TARGET} OUTPUT_NAME)
-    get_target_property(RUNTIME_OUTPUT_DIRECTORY ${TARGET} RUNTIME_OUTPUT_DIRECTORY)
     set(OUTPUT_FILE_NAME "${TARGET_OUTPUT_NAME}.${OUTPUT_EXTENSION}")
-    set(OUTPUT_FILE_PATH "${RUNTIME_OUTPUT_DIRECTORY}/${OUTPUT_FILE_NAME}")
+
+    get_target_property(RUNTIME_OUTPUT_DIRECTORY ${TARGET} RUNTIME_OUTPUT_DIRECTORY)
+    if(RUNTIME_OUTPUT_DIRECTORY)
+        set(OUTPUT_FILE_PATH "${RUNTIME_OUTPUT_DIRECTORY}/${OUTPUT_FILE_NAME}")
+    else()
+        set(OUTPUT_FILE_PATH "${OUTPUT_FILE_NAME}")
+    endif()
+
     add_custom_command(
         TARGET ${TARGET}
         POST_BUILD