discover: Add support for checking directories in parser API

discover: Add support for checking directories in parser API

This change adds a function to the parser API:

  int parser_check_dir(struct discover_context *ctx,
  		struct discover_device *dev, const char *dirname)

- which allows parsers to check for the presence of a directory (path of
'dirname') on the device ('dev'). We use this in the GRUB2 parser to
implement the `test -d` check.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>