boards: arduino_101: support 'make flash' over DFU

Let 'make flash' invoke the dfuutil.sh support script, and
export the relevant DFUUTIL_* environment variables, for all
3 cores of the Arduino 101 board: x86, ARC and BLE.

This is backward compatible with the current usage of OpenOCD
over JTAG, since this is only enabled when the environment
variable ZEPHYR_FLASH_OVER_DFU is set to y.

Change-Id: Ic5528cb87a180378d7120d150c27d1e24c9ebe75
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>