Improve signature check for UPDI parts

[stefanrueger]

Fixes #1813.

Microchip decided to put the signature area for UPDI at two different memory offsets, so you now need to know which part you have in order to be able to read the signature. This PR checks all different known offsets if an attempt to read a UPDI-part signature draws a blank. It's not fool-proof as, conceivably, if there was a valid signature at the wrong location that byte sequence would mistakenly be understood as the signature of the part.

Without this PR:

$ avrdude -cserialupdi -p64dd28 -t
avrdude: AVR device initialized and ready to accept instructions
avrdude: device signature = 0xffffff (probably .avr8x) (retrying)
avrdude: device signature = 0xffffff (probably .avr8x) (retrying)
avrdude: device signature = 0xffffff (probably .avr8x)
avrdude error: Yikes!  Invalid device signature.
avrdude error: expected signature for AVR64DD28 is 1E 96 1B
        Double check connections and try again, or use -F to override
        this check.


avrdude done.  Thank you.

With this PR:

$ avrdude -cserialupdi -p64dd28 -t
avrdude: AVR device initialized and ready to accept instructions
avrdude: device signature = 1E 96 22 (AVR64DU28)
avrdude error: expected signature for AVR64DD28 is 1E 96 1B
        double check chip or use -F to override this check

avrdude done.  Thank you.

The code graciously assumes Microchip won't create more than ten different regions for signatures within their UPDI series.