Add CONFIG_CDCACM_IFLOWCONTROL. Just hooks for now

drivers/usbdev/Kconfig

@@ -280,6 +280,14 @@ config CDCACM_CONSOLE
 		Register the USB device as /dev/console so that is will be used
 		as the console device.
 
+config CDCACM_IFLOWCONTROL
+	bool "CDC/ACM RTS flow control"
+	default n
+	depends on EXPERIMENTAL
+	select SERIAL_IFLOWCONTROL
+	---help---
+		Enable CDC/ACM RTS flow control
+
 config CDCACM_COMPOSITE
 	bool "CDC/ACM composite support"
 	default n
@@ -438,6 +446,7 @@ config CDCACM_VENDORSTR
 config CDCACM_PRODUCTSTR
 	string "Product string"
 	default "CDC/ACM Serial"
+
 endif
 
 config USBMSC

drivers/usbdev/cdcacm.c

@@ -2149,7 +2149,13 @@ static void cdcuart_rxint(FAR struct uart_dev_s *dev, bool enable)
 static bool cdcuart_rxflowcontrol(FAR struct uart_dev_s *dev,
                                   unsigned int nbuffered, bool upper)
 {
-#warning Missing logic
+#ifdef CONFIG_CDCACM_IFLOWCONTROL
+  /* Allocate a request */
+  /* Format the SerialControlLineState messages */
+  /* Submit the request on the Interrupt IN endpoint */
+#  warning Missing logic
+#endif
+
   return false;
 }
 #endif