scripts/nsdeps: support nsdeps for external module builds

scripts/nsdeps is written to take care of only in-tree modules.
Perhaps, this is not a bug, but just a design. At least,
Documentation/core-api/symbol-namespaces.rst focuses on in-tree modules.

Having said that, some people already tried nsdeps for external modules.
So, it would be nice to support it.

Reported-by: Steve French <smfrench@gmail.com>
Reported-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jessica Yu <jeyu@kernel.org>
Acked-by: Jessica Yu <jeyu@kernel.org>
Reviewed-by: Matthias Maennich <maennich@google.com>
Tested-by: Matthias Maennich <maennich@google.com>

Documentation/core-api/symbol-namespaces.rst

@@ -152,3 +152,6 @@ in-tree modules::
 	- notice the warning of modpost telling about a missing import
 	- run `make nsdeps` to add the import to the correct code location
 
+You can also run nsdeps for external module builds. A typical usage is::
+
+	$ make -C <path_to_kernel_src> M=$PWD nsdeps