fix arm64 iso build with `--bootloaders grub-efi`

!310 (754c4ad0) will fix this issue: https://www.mail-archive.com/debian-live@lists.debian.org/msg18830.html. This is aready merged in another pr

Device tree has better support for arm devices compared with acpi, so it's better to install dtbs to the image. This commit will be in another pr

I'm building iso with --bootloaders grub-efi, which will fail with E: iso-hybrid images not supported with as BIOS bootloader. So this error should only come with LB_BOOTLOADER_BIOS been set.