x86 netboot: use boot screen artwork
The current /debian-installer/amd64/grub/grub.cfg tries loading /isolinux/splash.png and /splash.png, neither of which can be found in netboot.tar.gz.
Create /splash.png in netboot.tar.gz as a symbolic link to /debian-installer/amd64/boot-screens/splash.png to make grub pick the splash screen when using netboot.
Tested by creating a new netboot image with make build_netboot
and PXE
booting it on a local VM with libvirt, TFTP server logs follow.
sarzana dnsmasq-tftp[4176]: file /srv/tftp/bookworm/isolinux/splash.png not found for 192.168.122.7 sarzana dnsmasq-tftp[4176]: sent /srv/tftp/bookworm/splash.png to 192.168.122.7
Bug: #1036215