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>
diff --git a/test/boot/bootdev.c b/test/boot/bootdev.c
index 23ebc61..c635d06 100644
--- a/test/boot/bootdev.c
+++ b/test/boot/bootdev.c
@@ -16,13 +16,6 @@
 #include <test/ut.h>
 #include "bootstd_common.h"
 
-/* Allow reseting the USB-started flag */
-#if defined(CONFIG_USB_HOST) || defined(CONFIG_USB_GADGET)
-extern bool usb_started;
-#else
-#include <usb.h>
-#endif
-
 /* Check 'bootdev list' command */
 static int bootdev_test_cmd_list(struct unit_test_state *uts)
 {
@@ -197,7 +190,7 @@
 	test_set_skip_delays(true);
 
 	/* Start up USB which gives us three additional bootdevs */
-	usb_started = false;
+	bootstd_reset_usb();
 	ut_assertok(run_command("usb start", 0));
 
 	/*
@@ -313,7 +306,7 @@
 	test_set_eth_enable(false);
 
 	/* Start up USB which gives us three additional bootdevs */
-	usb_started = false;
+	bootstd_reset_usb();
 	ut_assertok(run_command("usb start", 0));
 
 	ut_assertok(bootstd_test_drop_bootdev_order(uts));
@@ -352,7 +345,7 @@
 {
 	struct bootstd_priv *std;
 
-	usb_started = false;
+	bootstd_reset_usb();
 	test_set_skip_delays(true);
 
 	/* get access to the used hunters */
@@ -392,7 +385,7 @@
 	struct bootstd_priv *std;
 
 	test_set_skip_delays(true);
-	usb_started = false;
+	bootstd_reset_usb();
 
 	/* get access to the used hunters */
 	ut_assertok(bootstd_get_priv(&std));
@@ -520,7 +513,7 @@
 /* Check hunting for bootdev of a particular priority */
 static int bootdev_test_hunt_prio(struct unit_test_state *uts)
 {
-	usb_started = false;
+	bootstd_reset_usb();
 	test_set_skip_delays(true);
 
 	ut_assertok(bootdev_hunt_prio(BOOTDEVP_4_SCAN_FAST, false));
@@ -548,7 +541,7 @@
 	struct bootstd_priv *std;
 	int mflags;
 
-	usb_started = false;
+	bootstd_reset_usb();
 
 	/* get access to the used hunters */
 	ut_assertok(bootstd_get_priv(&std));