libfdt: Correct fdt handling of overlays without fixups and base trees without symbols

The fdt_overlay_apply() function purports to support the edge cases where
an overlay has no fixups to be applied, or a base tree which has no
symbols (the latter can only work if the former is also true).  However it
gets it wrong in a couple of small ways:

  * In the no fixups case, it doesn't fail immediately, but will attempt
    fdt_for_each_property_offset() giving -FDT_ERR_NOTFOUND as the node
    offset, which will fail.  Instead it should succeed immediately, since
    there's nothing to do.
  * In the case of no symbols, it again doesn't fail immediately.  However
    if there is an actual fixup it will fail with an unexpected error,
    because -FDT_ERR_NOTFOUND is passed to fdt_getprop() when attempting to
    look up the symbols.  We should instead return -FDT_ERR_NOTFOUND
    directly.

Both of these errors lead to the code returning misleading error codes in
failing cases.

[ DTC commit: 7d8ef6e1db9794f72805a0855f4f7f12fadd03d3 ]

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Simon Glass <sjg@chromium.org>

lib/libfdt/fdt_overlay.c

@@ -359,6 +359,9 @@ static int overlay_fixup_one_phandle(void *fdt, void *fdto,
 	int symbol_off, fixup_off;
 	int prop_len;
 
+	if (symbols_off < 0)
+		return symbols_off;
+
 	symbol_path = fdt_getprop(fdt, symbols_off, label,
 				  &prop_len);
 	if (!symbol_path)
@@ -492,7 +495,9 @@ static int overlay_fixup_phandles(void *fdt, void *fdto)
 
 	/* We can have overlays without any fixups */
 	fixups_off = fdt_path_offset(fdto, "/__fixups__");
-	if ((fixups_off < 0 && (fixups_off != -FDT_ERR_NOTFOUND)))
+	if (fixups_off == -FDT_ERR_NOTFOUND)
+		return 0; /* nothing to do */
+	if (fixups_off < 0)
 		return fixups_off;
 
 	/* And base DTs without symbols */