makefile.iotlab.single: add IoT-LAB testbed support for one node

Add support to do flash/reset/term on an IoT-LAB node.
It also allow running test using 'testrunner'.

Configuration variables are:

* `IOTLAB_NODE` which should be set to your node url
  * The full url including site to use from your computer `m3-1.grenoble.iot-lab.info`
  * The short url when used on the IoT-LAB frontend `m3-1`
* `IOTLAB_EXP_ID` for your experiment id for flash and reset.
  By default it tries to use your currently running experiment if you have only one
* `IOTLAB_USER`: is read from `${HOME}/.iotlabrc` as saved by `iotlab-auth`
  * It is expected to have run `iotlab-auth` beforehand.