dtc: Make dtc_open_file() die() if unable to open requested file

All current callers of dtc_open_file() immediately die() if it returns
an error.  In a non-interative tool like dtc, it's hard to see what
you could sensibly do to recover from a failure to open an input file
in any case.

Therefore, make dtc_open_file() itself die() if there's an error
opening the requested file.  This removes the need for error checking
at the callsites, and ensures a consistent error message in all cases.
While we're at it, change the rror message from fstree.c when we fail
to open the input directory to match dtc_open_file()'s error message.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

fstree.c

@@ -31,8 +31,8 @@ static struct node *read_fstree(const char *dirname)
 	struct node *tree;
 
 	d = opendir(dirname);
-	if (! d)
-		die("opendir(): %s\n", strerror(errno));
+	if (!d)
+		die("Couldn't opendir() \"%s\": %s\n", dirname, strerror(errno));
 
 	tree = build_node(NULL, NULL);