scripts: west-commands: Updates needed when removing ZEPHYR_BASE env

This commit includes the following fixes in order to remove environment
setting of ZEPHYR_BASE is west extension commands.

- Build command
  west build --pristine will now use the ZEPHYR_BASE variable found
  in CMakeCache.txt in the build folder.

  This ensures that the pristine command is executed from the same
  Zephyr that was used for compilation.

- Board command
  The west boards command no longer sets Zephyr base before invoking
  cmake -P cmake/boards.cmake

  Instead boards.cmake uses find_package(Zephyr) to ensure consistent
  behavior with Zephyr samples, so that the detection of Zephyr base is
  uniform across CMake commands.

  It also changes BOARD_ROOT_SPACE_SEPARATED to BOARD_ROOT in order to
  be consistent with existing user documentation.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>