PLEASE DON'T MERGE BEFORE TESTING > Support serial number on USB

[garnould]

Please test on Windows and Linux computer. Tested on OSX.

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Adding serial number support ('Each device has a unique 128-bit serial number which is a concatenation of four 32-bit words')

Based on chapter 8.8.3 of https://www.mouser.com/ds/2/268/Atmel-42223-SAM-R21_Datasheet-1065540.pdf

Computing an XOR'ed 32-bit value of ((uint32_t )(0x0080A00C)) ^ ((uint32_t )(0x0080A040)) ^ ((uint32_t )(0x0080A044)) ^ ((uint32_t )(0x0080A048))

• Displaying this value when using the newly created 's' command
• Using this value as USB device serial number when SAMR21E18A is define (-DSAMR21E18A)