Define the SystemClock_Config as a weak function.

This allow end user to redefine the clock configuration.

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

variants/NUCLEO_F030R8/variant.cpp

@@ -147,7 +147,7 @@ void init( void )
   * @param  None
   * @retval None
   */
-void SystemClock_Config(void)
+WEAK void SystemClock_Config(void)
 {
   RCC_ClkInitTypeDef RCC_ClkInitStruct;
   RCC_OscInitTypeDef RCC_OscInitStruct;