Add `f3brew` progress reporting

[darkk]

I've recently stumbled upon somewhat slow flash drive, that takes ~3 hours to go through f3brew cycle... But the flash drive is so cute and tiny that I decided to add a progressbar to f3brew instead of tossing the microSD-sized thumbdrive into trash bin :laughing:

Here is the patch adding a progressbar to f3brew.

Possible issue might come from fflush(stdout) for every written block of data as that might be somewhat slow on some incredibly slow terminals. On the other hand, I don't expect anyone to run f3brew over 9600 serial console and this behavior is also in sync with libflow.c, so this issue does not seem important to me.

Thanks for your awesome work and fighting the good fight.