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>
diff --git a/README b/README
index 91c3ac0..b0c2fdc 100644
--- a/README
+++ b/README
@@ -1367,6 +1367,13 @@
 			for your device
 			- CONFIG_USBD_PRODUCTID 0xFFFF
 
+		Some USB device drivers may need to check USB cable attachment.
+		In this case you can enable following config in BoardName.h:
+			CONFIG_USB_CABLE_CHECK
+			This enables function definition:
+			- usb_cable_connected() in include/usb.h
+			Implementation of this function is board-specific.
+
 - ULPI Layer Support:
 		The ULPI (UTMI Low Pin (count) Interface) PHYs are supported via
 		the generic ULPI layer. The generic layer accesses the ULPI PHY