adapted esp idf spiffs initialization

[OlliLenz]

Hi @matth-x,

If you use more than one spiffs partition in your project, it makes sense to adjust the .base_path of the partitions so that it is not just "/".
In the current version, however, this unfortunately leads to problems when using the esp-idf:

1. if you set the base_path to "/mo/", for example, you get an invalid argument error when initialising the spiffs partition, as the base path must not contain a slash at the end.
2. if you set the base_path to "/mo", the initialisation works, but the file names are no longer correct because they do not begin with a "/". An attempt is then made to save /mobootstats.jsn, for example, which does not work.

For this reason, a check has been introduced which, when setting the base_path, checks whether a "/" is present at the end of the define MOCPP_FILENAME_PREFIX. If this is the case, it is only removed for the base_path. Thus, the file names in the further course are also correct.

In addition, the define MOCPP_PARTITION_LABEL was introduced. The change from the former "ao" to "mo" could lead to problems in a production environment if devices are in circulation that have partitions with the name "ao", as these cannot or should not be changed OTA without further ado.