[NUCLEO_F302R8] Move back SetSysClock() call in SystemInit()

The serial_init() function is called before the mbed_sdk_init() (I don't
know why?) and this causes an incorrect baudrate calculation.

libraries/mbed/targets/cmsis/TARGET_STM/TARGET_NUCLEO_F302R8/system_stm32f30x.c

@@ -212,6 +212,10 @@ void SystemInit(void)
 #else
   SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH */
 #endif
+
+  /* Configure the System clock source, PLL Multiplier and Divider factors,
+     AHB/APBx prescalers and Flash settings */
+  SetSysClock();
 }
 
 /**

libraries/mbed/targets/hal/TARGET_STM/TARGET_NUCLEO_F302R8/mbed_overrides.c

@@ -29,10 +29,6 @@
 
 // This function is called after RAM initialization and before main.
 void mbed_sdk_init() {
-    /* Configure the System clock source, PLL Multiplier and Divider factors,
-     AHB/APBx prescalers and Flash settings */
-    SetSysClock();
-
     // Update the SystemCoreClock variable.
     SystemCoreClockUpdate();
 }