atmel: add a flashing helper and documentation

Provide a flashing helper to properly select the sam-ba configuration
and reuse the sam-ba tcl script provided by Atmel with minimal
modifications.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
3 files changed