Extract all inline systemd unit and drop-in content from install.go and
firstboot.go into a dedicated cmd/installer/systemd package for editability.
Bash scripts remain inline in firstboot.go.
Also copies the GRUB splash image from the live ISO into the installed
system's /boot/grub/ and sets GRUB_BACKGROUND so the branded background
appears in the post-install GRUB, not just the installer GRUB.
Julian Sommer