devtool: return specific exit code for incompatible recipes

Certain recipes cannot be used with devtool extract / modify / upgrade -
usually because they don't provide any source. Return a specific exit
code (4) so that scripts such as scripts/contrib/devtool-stress.py know
the difference between this and a genuine failure.

(From OE-Core rev: ffd295fed4ab81fc0bd00bb145ef4d72c49584bf)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/devtool

@@ -334,7 +334,7 @@ def main():
     except DevtoolError as err:
         if str(err):
             logger.error(str(err))
-        ret = 1
+        ret = err.exitcode
     except argparse_oe.ArgumentUsageError as ae:
         parser.error_subcommand(ae.message, ae.subcommand)