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>
diff --git a/scripts/west_commands/sign.py b/scripts/west_commands/sign.py
index 0aad0b2..26a5971 100644
--- a/scripts/west_commands/sign.py
+++ b/scripts/west_commands/sign.py
@@ -18,9 +18,9 @@
 from runners.core import BuildConfiguration
 from zcmake import CMakeCache
 from zephyr_ext_common import Forceable, cached_runner_config, \
-    zephyr_scripts_path
+    ZEPHYR_SCRIPTS
 
-sys.path.append(os.path.join(zephyr_scripts_path(), 'dts'))
+sys.path.append(str(ZEPHYR_SCRIPTS / 'dts'))
 
 import edtlib