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Thanks for your opinion, my technical question however remains unanswered. Do mean, it's no problem to change the PKGBUILD?
It's not my opinion. Replacing the original Linux kernel is the explicit purpose of the Linux-libre. It's the only reason it exists; there's no technical reason for it. Hence, the folks who wrote that PKGBUILD (three Arch developers) had no reason to believe one would not want to replace the stock kernel when installing the linux-libre package.
Ordinarily you could change the "provides=" and "conflicts=" variables in the PKGBUILD, so long as there's no actual conflict. If during installation the compiled kernel image is given its own name different from that of the stock kernel, and the kernel modules are installed to their own separate folder beside the folder containing modules installed by the stock kernel package, then you can simply remove the "conflicts" line and it should be fine. Otherwise pacman will complain about the files already existing and fail. From what I can tell from a quick glance PKGBUILD just builds a stock Arch kernel minus the binary blobs, so the PKGBUILD would need a somewhat extensive re-write in order to install both kernels side-by-side---again, because installing both side-by-side is not intended.
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