assemble.py: don't read BOARD.dts.pre.tmp

This file has been removed from upstream Zephyr in commit 2b7c61e306a
("cmake: re-work devicetree preprocessing steps").

Get the board name from .config instead; this is a stable place for it
to be found. Load the EDT itself from the pickle file in the build
directory; this has the advantage of fixing the script when out of
tree devicetree bindings are used.

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