prepare_boot_partitions_for_softraid() is used in BIOS and UEFI
modes to prepare the partitions for the bootloader. Move it from
the image extensions to raid_utils to reflect this and avoid the
import of an extension to efi_utils.
Follow-up to 62c5674a600baeeef0af3b12baeab486870eb103.
Change-Id: I9f5974fbbfea5e8cdfbb7e49bea375e5cbfdd145