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/video/readers/tga.c

@@ -297,7 +297,7 @@ grub_video_reader_tga (struct grub_video_bitmap **bitmap,
 
   grub_memset (&data, 0, sizeof (data));
 
-  data.file = grub_buffile_open (filename, 0);
+  data.file = grub_buffile_open (filename, GRUB_FILE_TYPE_PIXMAP, 0);
   if (! data.file)
     return grub_errno;