debugging: add a slide about OpenOCD

[clementleger]

OpenOCD is really poweful when debugging low-level kernel, maybe a slide and a lab might be really useful for kernel debugging.

[tpetazzoni]

We have a very short single slide on JTAG/OpenOCD in the kernel training, but it's definitely very short. When it comes to doing a lab, we should not forget about the impact on on-site training: if a separate JTAG probe is needed, then it should be planned as part of the HW to have available for the course. Of course if the JTAG interface is built into the board through some FTDI-like interface, it's totally fine as it would not require extra hardware.

[clementleger]

The ST-LINK debug interface is embedded on the board which makes it an ideal candidate for JTAG debugging actually :) No need for any additional hardware

[tpetazzoni]

Indeed, sounds perfect!