On fresh SBC images, the EFI partition has sd-boot UKI files but no
GRUB config. During upgrade, Probe() found sd-boot and used it, which
failed because RPi5/CM5 firmware lacks EFI SetVariableRT support.
Add arm64 guard to Probe(): when no GRUB config is found, skip sd-boot
probing and return a fresh GRUB config. This transitions from sd-boot
to GRUB on the first upgrade from a fresh flash.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Force GRUB instead of sd-boot on arm64 and pass --no-nvram to
grub-install, working around the SetVariableRT firmware limitation
that prevents in-place upgrades on RPi5/CM5 hardware.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
