Make LoadKernel() pass back the kernel partition's UniqueGuid.

LoadKernel already returns the partition number for the selected kernel.
This change makes it also return the GPT Entry's UniqueGuid, which will
eventually be passed to the kernel itself, so the kernel can determine which
of several possible devices it has booted from. It doesn't know for certain
because the BIOS and the kernel may enumerate the devices in a different
order.

BUG=chromium-os:4984

Review URL: http://codereview.chromium.org/3056014

firmware/lib/cgptlib/include/cgptlib_internal.h

@@ -112,4 +112,7 @@ int IsUnusedEntry(const GptEntry* e);
 /* Returns 1 if the entry is a Chrome OS kernel partition, else 0. */
 int IsKernelEntry(const GptEntry* e);
 
+/* Copies the current kernel partition's UniquePartitionGuid to the dest */
+void GetCurrentKernelUniqueGuid(GptData *gpt, void *dest);
+
 #endif /* VBOOT_REFERENCE_CGPTLIB_INTERNAL_H_ */