samd/mboot: Provide a UF2 bootloader for SAMD21 Xplained Pro.

A bootloader labelled for the SAMD21 XPLAINED PRO board.  The only
difference to a generic bootloader are the names and ID of the USB port and
the label of the drive that is opened.

Signed-off-by: robert-hh <[email protected]>

Author: robert-hh