https://libreboot.org/docs/gnulinux/grub_boot_installer.html
You can follow that link. When the libreboot menu shows up, pres "c" and set manually the device and roots for vmlinuz and initrd. From a USB, it should be something like this (each line is a command):
set root=‘usb0’ linux /install/vmlinuz initrd /install/initrd.gz boot
