zephyr: Find board name from dts.pre.tmp file

In scripts/assemble.py:

Use the <board_name>.dts.pre.tmp file in <build-dir>/zephyr to find the
board name.

This allows the build directory path to be set freely and not enforcing
directory structures.

Signed-off-by: Viktor Sjölind <viktor.sjolind@endian.se>