USB host composite: Add an option to permit support only a subset of the composite intefaces.

drivers/usbhost/Kconfig

@@ -87,6 +87,16 @@ config USBHOST_COMPOSITE
 		NOTE: This feature is marked EXPERIMENTAL because it has not been
 		untested
 
+config USBHOST_COMPOSITE_STRICT
+	bool "Strict composite membership"
+	default n
+	depends on USBHOST_COMPOSITE
+	---help---
+		If selected, then the composite device will not be enumerated unless
+		every member class in the composite is supported.  If not selected
+		then, for example, you could use the CDC/ACM interface of the device
+		with not support for the other interfaces.
+
 config USBHOST_MSC
 	bool "Mass Storage Class Support"
 	default n

drivers/usbhost/usbhost_composite.c

@@ -771,8 +771,12 @@ int usbhost_composite(FAR struct usbhost_hubport_s *hport,
       if (reg == NULL)
         {
           uerr("ERROR: usbhost_findclass failed\n");
+#ifdef CONFIG_USBHOST_COMPOSITE_STRICT
           ret = -EINVAL;
           goto errout_with_cfgbuffer;
+#else
+          continue;
+#endif
         }
 
       /* Yes.. there is a class for this device.  Get an instance of its