binman: Skip node generation for images read from files

We can and should run the node generator only when creating a new image.
When we read it back, there is no need to generate nodes - they already
exits, and binman does not dive that deep into the image - and there is
no way to provide the required fdt-list. So store the mode in the image
object so that Entry_fit can simply skip generator nodes when reading
them from an fdtmap.

This unbreaks all read-backs of images that contain generator nodes in
their fdtmap. To confirm this, add a corresponding test case.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Add SPDX to dts file:
Signed-off-by: Simon Glass <sjg@chromium.org>