Notes:
* Currently limited to 32-bit sector addresses due to internal Rockbox
APIs. So this means a practical limit of 2TiB per drive.
* Only 'General Data' GPT partition type is recognised, as that's
what SD cards seem to use for exFAT/FAT32.
Note that _booting_ off GPT-partitioned drive will require rebuilding
the various rockbox bootloaders, and even then there may be platform
limitations that preclude this.
Change-Id: Ibfaae1960adcb1e81976d4b60dd596c6d16318e4
