Add BOSSA.

BOSSA is a flash programming utility for Atmel's SAM family of flash-based ARM microcontrollers. The motivation behind
BOSSA is to create a simple, easy-to-use, open source utility to replace Atmel's SAM-BA software. BOSSA is an acronym
for Basic Open Source SAM-BA Application to reflect that goal.
Merge pull request #3809 from ambrop72/bossa

Add BOSSA.