scripts: assemble.py: switch to using devicetree_legacy_unfixed.h

Switch to using the devicetree_legacy_unfixed.h header for extracting
flash partition information.

Zephyr has switched to using a new gen_defines.py script and a new set
of macros/defines for devicetree code generation for all nodes except
flash partitions, which are still in the legacy format.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>