MFC r195045:
On special systems where the MBR and the GPT are in sync (up to the 4th
slicei, Apple EFI hardware), the bootloader will fail to recognize the GPT
if it finds anything else but the EFI partition. Change the check to continue
detecting the GPT by looking at the EFI partition on the MBR but
stopping successfuly after finding it.

PR: kern/134590
Submitted by: Christoph Langguth <christoph at rosenkeller.org>
Reviewed by: jhb