It turns out the warnings it spits out when forcing FAT32 with too few
clusters is not for nothing. At the very least the Raspberry Pi 3B does
not like the forced FAT 32. Though, it was validated that a large enough
FAT32 partition is fine with the Raspberry Pi 3B.
The factor was derived from first making a few test images, finding
there was a common thread, and then looking at what parts of FAT32 were
generated, and how they were generated. That was fun. (Yes, really.)
In addition, present the sector size as an option.
The sector size is required to set a block size in bytes, so giving the
option to the end-user is probably for the best.
The values here were chosen to keep the builder compatible with the
existing tests.
In addition, the values are quite optimized for smaller filesystem
images, e.g. building a FIRMWARE partition for the Raspberry Pi.
The current code in <nixpkgs> is reproducible since the date/time comes
from the store. With my particular tests, the date/time is the current
one since I'm using `>` to create test files.
