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>