Reference code for the SAMD2x disables caching in the NVM controller when
issuing NVM commands.  Let's do this as well to be consistent and safer.

Add a "chip-erase" for the Atmel SAMD targets that issues a complete Chip Erase
via the Device Service Unit (DSU).  This can be used to "unlock" or otherwise
unbrick a chip that can't be halted or inspected, allowing the user to reflash
with new firmware.

Add a "set-security" command which issues an SSB.  Once that's done and the
device is power-cycled, the flash cannot be written to until a "chip-erase" is
issued.  The chip-erase cannot be issued by openocd at this time because
the device will not respond to a request for the DAP IDCODE.

Change-Id: I80122f0bbf7e3aedffe052c1e77d69dc2dba25ed
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/2239


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>