autoboot: Support VOL-+ combo for fastboot

This is conditional:
In order to not cancel booting while trying to enter fastboot in android, the combo MUST be initiated with `VOL+` (hold) -> `VOL-` (hold)

bootloader/main.c

@@ -998,7 +998,7 @@ skip_list:
 	// Wait before booting. If VOL- is pressed go into bootloader menu.
 	if (!h_cfg.sept_run && !(b_cfg.boot_cfg & BOOT_CFG_FROM_LAUNCH))
 	{
-		btn = btn_wait_timeout(h_cfg.bootwait * 1000, BTN_VOL_DOWN);
+		btn = btn_wait_timeout(h_cfg.bootwait * 1000, BTN_VOL_DOWN | BTN_SINGLE);
 
 		if (btn & BTN_VOL_DOWN)
 			goto out;