'make prepare' nukes existing esp-idf directory

[Spritetm]

Seems that if you run 'make prepare', if you have an IDF_PATH set (most likely, could also be something else that is set by esp-idfs export.sh), the process nukes the esp-idf directory it points to. As an ESP-IDF developer, that makes me sad. (It luckily also makes me glad that I have working incremental backups.)

[robotman2412]

I'll have a look at it.