spiffs: fix build issue on clean project
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

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf