Improve get_SD_usage_data function

[robinkrahl]

• The NitrokeyManager::get_SD_usage_data function currently only returns the min/max write levels. It could also return the min/max read levels.

Also, the documentation should be improved:

• Intuitively, one would assume that the returned levels are in the range 0..100, but they are actually in the range 1..99. Is this intended?
• Is the returned range inclusive or exclusive?

[szszszsz]

Hi!
Good questions! Will check

[szszszsz]

According to the Nitrokey Storage firmware, the reads are exclusive, hence the values range probably [1].

In my understanding use case is to provide minimum/maximum range, where the data could be safely stored, thus the +1/-1 from the limits. E.g. there are some writes in the beginning of the SD card, in the very first percentile, region 0%-1%, so the value returned will be 1%. Value 0% would not have meaning here.
Maybe could be useful to show that the write has not been executed in this power cycle yet, but this will bring unnecessary complexity.

[1] https://github.com/Nitrokey/nitrokey-storage-firmware/blob/f66b2e63582312717b7e45ce078b3b8fd869c51d/src/OTP/report_protocol.c#L2658