bootstd: Create a function to reset USB

Set up a function for this, since it needs to be used from multiple test
files.

This test file is only used on sandbox, where USB is enabled, so drop
the local declaration of usb_started

Signed-off-by: Simon Glass <sjg@chromium.org>

test/boot/bootstd_common.h

@@ -53,4 +53,12 @@ int bootstd_setup_for_tests(void);
  */
 int bootstd_test_check_mmc_hunter(struct unit_test_state *uts);
 
+/**
+ * bootstd_reset_usb() - Reset the USB subsystem
+ *
+ * Resets USB so that it can be started (and scanning) again. This is useful in
+ * tests which need to use USB.
+ */
+void bootstd_reset_usb(void);
+
 #endif