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

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>