Add ability to compute last byte checksum (for Dallas 1990 format)

I'm not sure about all formats, but it looks as though the Dallas iButton protocol uses the last byte as a checksum and it should be computed using the first 7 bytes of the UID.

Could you please modify the iButton fuzzer so it doesn't add bogus data for the last byte (at least for Dallas protocol) and instead compute the last byte by using the values of the first 7 bytes.

If you try to manually add a Dallas 1990 key in Flipper and you edit any of the first 7 bytes you can see the last (rightmost) byte change value every time you change any of the other bits, seems to be a checksum of sorts.

I think the fuzzer should only mess with the first 7 bytes and automatically compute the last one.