Update latency tester

* Enable operation with all sources
* Measure strobe length on low-persistence displays

software/sys_controller/memory/sdcard.c

@@ -31,11 +31,8 @@ int check_sdcard(alt_u8 *databuf)
 
     res = SD_Init(&sdcard_dev);
     printf("SD det status: %u\n", res);
-    if (res) {
-        sniprintf(menu_row1, LCD_ROW_LEN+1, "No SD card det.");
-        menu_row2[0] = '\0';
-        return 1;
-    }
+    if (res != SD_OK)
+        return res;
 
     return SD_Read(&sdcard_dev, databuf, 0, 0, 512);
 }