[fprint] vfs101 driver and nr_enroll_stages

[Vasily Khoruzhick]

Hi Sergio,

I'm working on improvement for libfprint enrolling algorithm for
imaging devices;
basic idea is to scan same finger several times during enrollment and
store all scans for later usage.
It dramatically improves match rate on devices with small sensor
(AES1660, Upek 147e:2020, AES1610).

I've noticed that you're setting nr_enroll_stages in your vfs101
driver to 3, is there any reason for that?
Does vfs101 device assemble 3 images into one, or something like that?

My current version is on github:
https://github.com/anarsoul/libfprint/ branch anarsoul-wip
It would be nice if you can test it with your device and provide some
feedback/fixes.

Regards
Vasily