Basic image driver foundations

The basic model is that image drivers declare a fp_img_driver structure
rather than a fp_driver struct. fp_img_driver will contain primitive imaging
operations such as 'scan finger and return image'. The imgdev layer will
generically implement the primitive fp_driver operations, and the imgdev
layer will fix up the enroll/verify/etc pointers at driver registration
time.

Removed const from all fp_driver declarations, as these are now modified
dynamically in the case of imaging drivers.

libfprint/drivers/upekts.c

@@ -774,7 +774,7 @@ static const struct usb_id id_table[] = {
 	{ 0, 0, 0, }, /* terminating entry */
 };
 
-const struct fp_driver upekts_driver = {
+struct fp_driver upekts_driver = {
 	.name = FP_COMPONENT,
 	.full_name = "UPEK TouchStrip",
 	.id_table = id_table,