doc: arduino 101: adapt documentation for DFU flashing

The Arduino 101 board in Zephyr now support the original bootloader
and thus supports flashing over DFU. Adapt documentation and remove
reference to alternate bootloader which has been causing issues
and does not support flashing of the bluetooth firmware.

Jira: ZEP-301
Change-Id: I0298d8938bb636319b213909594d8cb42417559a
Signed-off-by: Anas Nashif <anas.nashif@intel.com>