usb: Tidy up the usb_start flag

This should be declared in a header file so that type-checking works
correctly.

Add a single declaration to usb.h and remove the others.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>

test/boot/bootdev.c

@@ -19,9 +19,9 @@
 
 /* Allow reseting the USB-started flag */
 #if defined(CONFIG_USB_HOST) || defined(CONFIG_USB_GADGET)
-extern char usb_started;
+extern bool usb_started;
 #else
-char usb_started;
+#include <usb.h>
 #endif
 
 /* Check 'bootdev list' command */