[STM32F4][V1.12.0] SystemCoreClock update

With ARM and uARM toolchains, SystemCoreClock is reset before UART init by libc.
Fix to perform SystemCoreClock init with UART init out of HAL init.

Author: Erwan GOURIOU

hal/targets/hal/TARGET_STM/TARGET_STM32F4/serial_api.c

@@ -182,6 +182,10 @@ static void init_uart(serial_t *obj, UARTName instance)
     }
 #endif
 
+    /* uAMR & ARM: Call to UART init is done between reset of pre-initialized variables */
+	/* and before HAL Init. SystemCoreClock init required here */
+    SystemCoreClockUpdate();
+
     if (HAL_UART_Init(handle) != HAL_OK) {
         error("Cannot initialize UART\n");
     }