Improve iSerial number

[jans23]

For Nitrokey Pro:
Currently the iSerial number (of USB interface) is something like 000000000000000000005F11. The leading part are always zeros while the actual App ID in this case is D276000124010303000500005F110000. Either the leading zeros should be dropped (so that a serial number of 8 characters is remaining) or replaced with the actual App ID.

For Nitrokey HSM:
I suggest to use everything behind DENK as a serial number, e.g. 0101774, and don't append any zeros. The iSerial would be seven digits long.