cmake: using ${ZEPHYR_BASE} instead of $ENV{ZEPHYR_BASE}

With the introduction of ZephyrConfig.cmake all parts of CMake code
should rely on the CMake ZEPHYR_BASE variable instead of the environment
setting.

This ensures that after the first CMake invocation, then all subsequent
invocation in same build folder will use same zephyr base.

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