cfg: add initial Atmel xplained kit support

cfg: add initial Atmel xplained kit support

These kits feature a CMSIS-DAP compliant debugger and so have been added
as part of the pending support.

Currently the flash drivers for the L8 and D20 are wip.

One issue this implementation of CMSIS-DAP raised is that it supports
512byte HID reports, however using the current HIDAPI we have no cross platform
way of querying this info. Long term we plan to add this support to HIDAPI.

Change-Id: Ie8b7c871f58a099d963cd71a9f8a0105a38784e9
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1625
Tested-by: jenkins