boards/iotlab-m3|a8: fix openocd configuration

iotlab-m3 boards always ended up not being able to flash after time.
This changes managed to fix and flash boards that where able to be flashed with
the deprecated `ft2232` driver and not with the `ftdi` driver used in RIOT.

It combines configuration from openocd, iot-lab, RIOT config and Alexandre
Abadie feedback

* http://repo.or.cz/openocd.git/blob/HEAD:/tcl/interface/ftdi/iotlab-usb.cfg
  * ftdi configuration
* https://github.com/iot-lab/iot-lab-gateway/blob/2.4.1/gateway_code/static/iot-lab-m3.cfg
  * `trst_and_srst` config
* Alexandre feedback and http://openocd.org/doc/html/Reset-Configuration.html
  * 'connect_assert_srst' reset configuration
    * it prevents errors in the output on first flash
    * should help on boards with invalid code
    * It was taken from what Alexandre found for board 'b-l072z-lrwan1'
  * It requires using '-c reset halt' instead of '-c halt' before debug
* RIOT
  * Keep the `configure -rtos` auto