Using find_package to locate Zephyr.
Old behavior was to use $ENV{ZEPHYR_BASE} for inclusion of boiler plate
code.
Whenever an automatic run of CMake happend by the build system / IDE
then it was required that ZEPHYR_BASE was defined.
Using ZEPHYR_BASE only to locate the Zephyr package allows CMake to
cache the base variable and thus allowing subsequent invocation even
if ZEPHYR_BASE is not set in the environment.
It also removes the risk of strange build results if a user switchs
between different Zephyr based project folders and forgetting to reset
ZEPHYR_BASE before running ninja / make.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
