hackrf_sweep: add multi-fft mode

[gozu42]

(mainly a tracking issue to keep my thoughts/code and potential feedback on this in one place, not sure i can tag it enhancement myself)

currently hackrf_sweep does only one fft per data block received. this means for the default 1MHz binwidth it uses a total of 40 bytes out of each 16kB block.

reporting modes for the results that come to mind:

• raw (output each fft result as a separate data row)
• min
• avg
• max

perhaps an option to change where in the data block samples are picked from. currently it is always the tail end, for multi-fft a sane default would be spread out evenly starting from the end, but something like "compact at the end" or even "overlapping" would be trivial to add while at it.

raw mode would also require some changes to the way the timestamp is generated. currently it is usb_transfer_time PLUS something, it should be MINUS to begin with and needs different way to handle on which block in the current transfer we are, and fine tuning for the multi-fft part.

unsure how do handle this in ifft mode, mainly because i dont understand that one to begin with.