Fix assemble.py tool for compatibility

This accounts for changes in the Zephyr build which include:

1) Flash areas are prefixed with `DT_`
2) The generated DTS was renamed to generated_dts_board_unfixed.h
3) The build tree has an extra "zephyr" dir.

Fixes #408

Signed-off-by: Fabio Utzig <utzig@apache.org>