Correct check for --sdk option (#2870)

Author: Julian-O

pythonforandroid/bootstraps/common/build/build.py

@@ -1006,7 +1006,7 @@ def _read_configuration():
         print('Billing not yet supported!')
         sys.exit(1)
 
-    if args.sdk_version == -1:
+    if args.sdk_version != -1:
         print('WARNING: Received a --sdk argument, but this argument is '
               'deprecated and does nothing.')
         args.sdk_version = -1  # ensure it is not used