IoT-LAB testbed-support enhancements

[cladmi]

List of things that I would like to improve on the IoT-LAB integration with IOTLAB_NODE.

https://github.com/RIOT-OS/RIOT/blob/master/dist/testbed-support/makefile.iotlab.single.inc.mk tool. W

• [x] Handle native to ignore IOTLAB_NODE. No running everything with IOTLAB_NODE=auto-ssh just puts a warning. (#16501)
• [x] Handle tools not installed (#16501)
• [ ] Factor things in common with the other iotlab script.
• [x] Debugger support. (#16510)
• [x] Handle putting IOTLAB_NODE as saclay,m3,1 as done in IoT-LAB scripts by default. (#16501)
• [x] Use IOT_LAB_FRONTEND_FQDN == SITE.iot-lab.info to know if executed from the frontend (#16491)

[cladmi]

Handle putting IOTLAB_NODE as saclay,m3,1 as done in IoT-LAB scripts by default.

[cladmi]

Now IoT-LAB frontends have an IOT_LAB_FRONTEND_FQDN == SITE.iot-lab.info env variable that can be used to know if executed from a frontend.

It would remove the need to have different auto and auto-ssh behavior and clean internal implementation.