Feature Request: blackbox managment

It will be very handful if will be possible to check black box occupied/free space and purge the data.

The issue does not name a file, script, entry point, or test. Start by locating the Lua scripts that communicate with Betaflight and inspect how blackbox storage is currently exposed. Clarify how occupied/free space should be reported and what purging should affect; done means those checks and the purge operation work through the supported transmitter interface.

Written by the indexing model from the issue text.