spiffs: fix build issue on clean project

From issue:

I did some poking around in components/spiffs/project_include.cmake and
it looks like what is failing is it isn't getting an offset from
parttool.py. I think what the problem is that the parttool.py cannot
determine the offset of the storage partition as I can't run menuconfig
to set the custom partitions.

I have tested that this problem is solvable by manually setting the
custom partitions in sdkconfig and that is a workaround but there should
be at least a better error message in the configuration when it cannot
find the partition as this would also happen if the partition name was
misspelled.

Closes https://github.com/espressif/esp-idf/issues/4236

components/spiffs/project_include.cmake

@@ -15,40 +15,46 @@ function(spiffs_create_partition_image partition base_dir)
     partition_table_get_partition_info(size "--partition-name ${partition}" "size")
     partition_table_get_partition_info(offset "--partition-name ${partition}" "offset")
 
-    set(image_file ${CMAKE_BINARY_DIR}/${partition}.bin)
-
-    if(CONFIG_SPIFFS_USE_MAGIC)
-        set(use_magic "--use-magic")
-    endif()
-
-    if(CONFIG_SPIFFS_USE_MAGIC_LENGTH)
-        set(use_magic_len "--use-magic-len")
-    endif()
-
-    if(CONFIG_SPIFFS_FOLLOW_SYMLINKS)
-        set(follow_symlinks "--follow-symlinks")
-    endif()
-
-    # Execute SPIFFS image generation; this always executes as there is no way to specify for CMake to watch for
-    # contents of the base dir changing.
-    add_custom_target(spiffs_${partition}_bin ALL
-        COMMAND ${spiffsgen_py} ${size} ${base_dir_full_path} ${image_file}
-        --page-size=${CONFIG_SPIFFS_PAGE_SIZE}
-        --obj-name-len=${CONFIG_SPIFFS_OBJ_NAME_LEN}
-        --meta-len=${CONFIG_SPIFFS_META_LENGTH}
-        ${follow_symlinks}
-        ${use_magic}
-        ${use_magic_len}
-        DEPENDS ${arg_DEPENDS}
-        )
-
-    set_property(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}" APPEND PROPERTY
-        ADDITIONAL_MAKE_CLEAN_FILES
-        ${image_file})
-
-    if(arg_FLASH_IN_PROJECT)
-        esptool_py_flash_project_args(${partition} ${offset} ${image_file} FLASH_IN_PROJECT)
+    if("${size}" AND "${offset}")
+        set(image_file ${CMAKE_BINARY_DIR}/${partition}.bin)
+
+        if(CONFIG_SPIFFS_USE_MAGIC)
+            set(use_magic "--use-magic")
+        endif()
+
+        if(CONFIG_SPIFFS_USE_MAGIC_LENGTH)
+            set(use_magic_len "--use-magic-len")
+        endif()
+
+        if(CONFIG_SPIFFS_FOLLOW_SYMLINKS)
+            set(follow_symlinks "--follow-symlinks")
+        endif()
+
+        # Execute SPIFFS image generation; this always executes as there is no way to specify for CMake to watch for
+        # contents of the base dir changing.
+        add_custom_target(spiffs_${partition}_bin ALL
+            COMMAND ${spiffsgen_py} ${size} ${base_dir_full_path} ${image_file}
+            --page-size=${CONFIG_SPIFFS_PAGE_SIZE}
+            --obj-name-len=${CONFIG_SPIFFS_OBJ_NAME_LEN}
+            --meta-len=${CONFIG_SPIFFS_META_LENGTH}
+            ${follow_symlinks}
+            ${use_magic}
+            ${use_magic_len}
+            DEPENDS ${arg_DEPENDS}
+            )
+
+        set_property(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}" APPEND PROPERTY
+            ADDITIONAL_MAKE_CLEAN_FILES
+            ${image_file})
+
+        if(arg_FLASH_IN_PROJECT)
+            esptool_py_flash_project_args("${partition}" "${offset}" "${image_file}" FLASH_IN_PROJECT)
+        else()
+            esptool_py_flash_project_args("${partition}" "${offset}" "${image_file}")
+        endif()
     else()
-        esptool_py_flash_project_args(${partition} ${offset} ${image_file})
+        set(message "Failed to create SPIFFS image for partition '${partition}'. "
+                    "Check project configuration if using the correct partition table file.")
+        fail_at_build_time(spiffs_${partition}_bin "${message}")
     endif()
 endfunction()