verifiers: File type for fine-grained signature-verification controlling

Let's provide file type info to the I/O layer. This way verifiers
framework and its users will be able to differentiate files and verify
only required ones.

This is preparatory patch.

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>

grub-core/commands/acpi.c

@@ -635,7 +635,7 @@ grub_cmd_acpi (struct grub_extcmd_context *ctxt, int argc, char **args)
       grub_size_t size;
       char *buf;
 
-      file = grub_file_open (args[i]);
+      file = grub_file_open (args[i], GRUB_FILE_TYPE_ACPI_TABLE);
       if (! file)
 	{
 	  free_tables ();