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/kern/dl.c

@@ -724,7 +724,7 @@ grub_dl_load_file (const char *filename)
      opens of the same device.  */
   grub_file_close (file);
 
-  grub_tpm_measure(core, size, GRUB_TPM_PCR, filename);
+  grub_tpm_measure(core, size, GRUB_BINARY_PCR, "grub_module", filename);
 
   mod = grub_dl_load_core (core, size);
   grub_free (core);