Describe PMBR in terms of syslinux GPT extension.

boot.c

Index: boot.c
diff --git a/boot.c b/boot.c
index dc34961467601b4d257184ee2d7700df151773c6..3abc272016b0ccc1f79184c72bd6a34a889237e1 100644
--- a/boot.c
+++ b/boot.c
@@ -179,8 +179,9 @@ boot(int fd)
                 warnx("unable to find partition to make bootable.");
                 return;
         }
-        le_uuid_enc(((unsigned char*)mbr + 424),
+        le_uuid_enc(&mbr->boot_guid,
                     &((struct gpt_ent*)(gptboot->map_data))->ent_uuid);
+        mbr->magic_number = htole16(MBR_MAGIC);
 #endif
 	gpt_write(fd, pmbr);