Convert USE_READLINE config option to be consistent with others.
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index 36cf138..7a4622b 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -1,5 +1,10 @@
// options to control how Micro Python is built
+// Linking with GNU readline causes binary to be licensed under GPL
+#ifndef MICROPY_USE_READLINE
+#define MICROPY_USE_READLINE (1)
+#endif
+
#define MICROPY_ENABLE_FLOAT (1)
#define MICROPY_EMIT_CPYTHON (0)
#define MICROPY_EMIT_X64 (1)