scripts: fix west sign when ZEPHYR_BASE is unset

Due to cleanups in west targeted at getting rid of zephyr-specific
code, extension commands can no longer rely on ZEPHYR_BASE being set
in the calling environment at import time (it's still set at run()
time for now, though, to keep west build working).

Add a new helper to make dealing with this easier from west sign.

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

scripts/west_commands/sign.py

@@ -17,11 +17,11 @@ from build_helpers import find_build_dir, is_zephyr_build, \
     FIND_BUILD_DIR_DESCRIPTION
 from runners.core import BuildConfiguration
 from zcmake import CMakeCache
-from zephyr_ext_common import Forceable, cached_runner_config
+from zephyr_ext_common import Forceable, cached_runner_config, \
+    zephyr_scripts_path
+
+sys.path.append(os.path.join(zephyr_scripts_path(), 'dts'))
 
-# FIXME we should think of a nicer way to manage sys.path
-# for shared Zephyr code.
-sys.path.append(os.path.join(os.environ['ZEPHYR_BASE'], 'scripts', 'dts'))
 import edtlib
 
 SIGN_DESCRIPTION = '''\
@@ -388,6 +388,5 @@ class RimageSigner(Signer):
                      ['-o', out_bin, '-m', 'apl', '-i', '3'] +
                      [bootloader, kernel])
 
-
         log.inf(quote_sh_list(sign_base))
         subprocess.check_call(sign_base)

scripts/west_commands/zephyr_ext_common.py

@@ -8,6 +8,9 @@ Note that common helpers used by the flash and debug extension
 commands are in run_common -- that's for common code used by
 commands which specifically execute runners.'''
 
+import os
+from pathlib import Path
+
 from west import log
 from west.commands import WestCommand
 
@@ -52,3 +55,12 @@ def cached_runner_config(build_dir, cache):
                         elf_file, hex_file, bin_file,
                         gdb=gdb, openocd=openocd,
                         openocd_search=openocd_search)
+
+# FIXME we should think of a nicer way to manage sys.path
+# for shared Zephyr code.
+def zephyr_scripts_path():
+    # This relies on this file being zephyr/scripts/foo/bar.py.
+    zephyr_base = Path(os.environ.get('ZEPHYR_BASE',
+                       Path(__file__).parent.parent.parent))
+
+    return str(zephyr_base / 'scripts')