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

@@ -15,6 +15,7 @@ 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")
 
+    if("${size}" AND "${offset}")
         set(image_file ${CMAKE_BINARY_DIR}/${partition}.bin)
 
         if(CONFIG_SPIFFS_USE_MAGIC)
@@ -47,8 +48,13 @@ function(spiffs_create_partition_image partition base_dir)
             ${image_file})
 
         if(arg_FLASH_IN_PROJECT)
-        esptool_py_flash_project_args(${partition} ${offset} ${image_file} FLASH_IN_PROJECT)
+            esptool_py_flash_project_args("${partition}" "${offset}" "${image_file}" FLASH_IN_PROJECT)
         else()
-        esptool_py_flash_project_args(${partition} ${offset} ${image_file})
+            esptool_py_flash_project_args("${partition}" "${offset}" "${image_file}")
+        endif()
+    else()
+        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()