usb: ums: add ums exit feature by ctrl+c or by detach usb cable

This patch allows exiting from UMS mode to u-boot prompt
by detaching usb cable or by pressing ctrl+c.

Add new config: CONFIG_USB_CABLE_CHECK. If defined then board
file should provide function: usb_cable_connected() (include/usb.h)
that return 1 if cable is connected and 0 otherwise.

Changes v2:
- add a note to the README

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Marek Vasut <marex@denx.de>

include/usb.h

@@ -197,6 +197,16 @@ int board_usb_init(int index, enum usb_init_type init);
  */
 int board_usb_cleanup(int index, enum usb_init_type init);
 
+/*
+ * If CONFIG_USB_CABLE_CHECK is set then this function
+ * should be defined in board file.
+ *
+ * @return 1 if cable is connected and 0 otherwise.
+ */
+#ifdef CONFIG_USB_CABLE_CHECK
+int usb_cable_connected(void);
+#endif
+
 #ifdef CONFIG_USB_STORAGE
 
 #define USB_MAX_STOR_DEV 5