labs: sysdev-buildroot: use Linux 6.4 headers

[Taumille]

In crosstool-ng 1.26.0, the kernel headers version used can't be higher than Linux 6.4.
Telling Buildroot that the toolchain was built with 6.6 headers will cause it to crash with this error message :
"Incorrect selection of kernel headers: expected 6.6.x, got 6.4.x"

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[Taumille]

I have read the CLA Document and I hereby sign the CLA

[tpetazzoni]

Hello @Taumille: it was in fact not necessary for you to sign the CLA, as you are a Bootlin employee. The CLA is only relevant for non-Bootlin people.

[tpetazzoni]

I've applied, but the fix is in fact not 100% correct. Indeed, while on BeagleBone, STM32MP1 and BeaglePlay we use the 6.6 kernel, on the Qemu board, we're still using the 6.1 kernel. Which means that the Crosstool-NG toolchain should be created with 6.1 kernel headers (or older) and then when Buildroot uses this toolchain, it will encounter 6.1 kernel headers (or older) and not 6.4.

The version of the kernel really running on the target is defined by the \workingkernel LaTeX macro. Maybe we need to define a \kernelheadersversion macro as well for each target, and use it in both the sysdev-toolchain.tex and sysdev-buildroot.tex labs, to make sure it remains consistent over-time?

(And then of course, it would be nice to update the Qemu target to also use the 6.6 kernel)