L2SDP-516, repaired bug in making version string.

src/periph/fpga.cpp

@@ -988,7 +988,7 @@ bool Periph_fpga::read_firmware_version(TermOutput& termout, int format)
     if (retval == false) { return retval; }
     string revision = to_string(data[0]);
 
-    version = date.substr(0, 2) + "-" + date.substr(2, 2) + "-" + date.substr(4, 2)
+    version = date.substr(0, 4) + "-" + date.substr(4, 2) + "-" + date.substr(6, 2)
             + "T"
             + time.substr(0, 2) + "." + time.substr(2, 2) + "." + time.substr(4, 2)
             + "_"