arm64/efi/linux: Ignore FDT unless we need to modify it

arm64/efi/linux: Ignore FDT unless we need to modify it

Now that we implemented support for the LoadFile2 protocol for initrd
loading, there is no longer a need to pass the initrd parameters via
the device tree. This means that when the LoadFile2 protocol is being
used, there is no reason to update the device tree in the first place,
and so we can ignore it entirely.

The only remaining reason to deal with the devicetree is if we are
using the "devicetree" command to load one from disk, so tweak the
logic in grub_fdt_install() to take that into account.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>