dts: marshal the EDT object for later use

We need to save and restore the devicetree data to generate optimized
dependency information later on in the build, in particular during the
final application link.

Make this happen by pickling the EDT object in BUILD_DIR/edt.pickle.

The existence of this file is an implementation detail, so do not add
it to the documentation.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
2 files changed