Recognise GRUB2 core.img (#766989)

Recognise GRUB2 core.img boot code written to a partition without a file
system.  Such setups are possible/likely with GPT partitioned disks as
there is a specific partition type reserved for it [1][2]:
    21686148-6449-6E6F-744E-656564454649  (BIOS Boot partition)

[1] GUID Partition Table, Partition types
    https://en.wikipedia.org/wiki/GUID_Partition_Table#Partition_type_GUIDs

[2] BIOS boot partition
    https://en.wikipedia.org/wiki/BIOS_boot_partition

Bug 766989 - zfsonline support - need file system name support for ZFS
             type codes