cmake: scripts: now using ZEPHYR_BASE as local variable

This commit is a followup to the usage of `find_package(Zephyr ...)`.

The zephyr/hello-world sample has been updated to use find_package.
The assemble.py script now takes ZEPHYR_BASE as an argument, so it may
be used from CMakeLists.txt files when ZEPHYR_BASE is not set in
environment, and thus the Makefile sample has been adjusted accordingly.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>