Rework TPM measurements

Rework TPM measurements to use fewer PCRs. After discussion with upstream,
it's preferable to avoid using so many PCRs. Instead, measure into PCRs 8
and 9 but use a prefix in the event log to indicate which subsystem carried
out the measurements.

grub-core/loader/i386/multiboot_mbi.c

@@ -165,7 +165,7 @@ grub_multiboot_load (grub_file_t file, const char *filename)
       return grub_errno;
     }
 
-  grub_tpm_measure((unsigned char*)buffer, len, GRUB_KERNEL_PCR, filename);
+  grub_tpm_measure((unsigned char*)buffer, len, GRUB_BINARY_PCR, "grub_multiboot", filename);
 
   header = find_header (buffer, len);