main: downgrade "no zephyr" to wrn(), add workaround

We're getting increasing interest in west being its own tool, to be
used standalone from Zephyr. Unfortunately we are still too tightly
coupled for that to be possible, but the current behavior of erroring
out is not correct. Let's downgrade that to a warning and have the
warning print the current workaround.

Relates to: #246
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

src/west/main.py

@@ -452,9 +452,14 @@ def set_zephyr_base(args):
                         zb_origin = 'manifest file {}'.format(manifest.path)
                         break
                 else:
-                    log.err('no --zephyr-base given, ZEPHYR_BASE is unset,',
-                            'west config contains no zephyr.base setting,',
-                            'and no manifest project has path "zephyr"')
+                    log.wrn("can't find the zephyr repository:\n"
+                            '  - no --zephyr-base given\n'
+                            '  - ZEPHYR_BASE is unset\n'
+                            '  - west config contains no zephyr.base setting\n'
+                            '  - no manifest project has path "zephyr"\n'
+                            "If this isn't a Zephyr installation, you can "
+                            "silence this warning with something like this:\n"
+                            '  west config zephyr.base not-using-zephyr')
             except MalformedConfig as e:
                 log.wrn("Can't set ZEPHYR_BASE:",
                         'parsing of manifest file failed during command',