stmhal/boards/STM32F7DISC: Define LSE_STARTUP_TIMEOUT.

So it compiles with the latest F7 hal.

stmhal/boards/STM32F7DISC/stm32f7xx_hal_conf.h

@@ -140,6 +140,10 @@
  #define LSE_VALUE  ((uint32_t)32768)    /*!< Value of the External Low Speed oscillator in Hz */
 #endif /* LSE_VALUE */
 
+#if !defined  (LSE_STARTUP_TIMEOUT)
+  #define LSE_STARTUP_TIMEOUT    ((uint32_t)5000U)   /*!< Time out for LSE start up, in ms */
+#endif /* LSE_STARTUP_TIMEOUT */
+
 /**
   * @brief External clock source for I2S peripheral
   *        This value is used by the I2S HAL module to compute the I2S clock source