Fixes <https://bugs.gnu.org/40538>.
Machines without Kernel Mode Setting (those with many old and current AMD
GPUs, SiS GPUs, …) need uvesafb to show the GUI installer. Some may also need
a kernel parameter like nomodeset or vga=793, but we leave that for the user
to specify in GRUB.
* gnu/system/install.scm (uvesafb-shepherd-service): New procedure.
(uvesafb-service-type): New variable.
(%installation-services): Add it.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>
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