usdr_flash: Display serial number

usdr's firmware contains 128-bit serial number on offset 0x3ff000.

`usdr_flash` should show this number in the following format:

5827-a08b-9a96-25d8-129a-5a7e-fa9c-881e

The SN must not be shown if all bytes is 00 or FF, which means usdr lacks SN.