doc: arduino_101: document 'make flash' over DFU

Explain how to create the udev rules granting access to the
Arduino 101 board in DFU mode.

Explain the available methods to flash the Arduino 101 board:
either manually with dfu-util command line,
or automated with ZEPHYR_FLASH_OVER_DFU=y and 'make flash'.

Provide instructions for x86, ARC and BLE cores, using distinct
code-blocks for the manual and make-assisted methods.

Change-Id: I0f9fe3849dec3c2dc2249b77d31d4f2414c98331
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>