Revert couple of checksum -> digest renames

- physical cpio header and elf header have checksums, not digests
  (from commit 7acdfded4a82e86a673418c3953aa6bf8106d313)
- doesn't affect functionality though

tools/debugedit.c

@@ -1396,7 +1396,7 @@ handle_build_id (DSO *dso, Elf_Data *build_id,
     if (elf64_xlatetom (&x, &x, dso->ehdr.e_ident[EI_DATA]) == NULL)
       {
       bad:
-	fprintf (stderr, "Failed to compute header digest: %s\n",
+	fprintf (stderr, "Failed to compute header checksum: %s\n",
 		 elf_errmsg (elf_errno ()));
 	exit (1);
       }