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.c

@@ -11,6 +11,7 @@
 #include <dm.h>
 #include <memalign.h>
 #include <mmc.h>
+#include <usb.h>
 #include <linux/log2.h>
 #include <test/suites.h>
 #include <test/ut.h>
@@ -88,6 +89,11 @@ int bootstd_test_check_mmc_hunter(struct unit_test_state *uts)
 	return 0;
 }
 
+void bootstd_reset_usb(void)
+{
+	usb_started = false;
+}
+
 int do_ut_bootstd(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
 	struct unit_test *tests = UNIT_TEST_SUITE_START(bootstd_test);