Make sure defining/not defining USE_UNDERCLOCK doesn't break the functionality

src/main/drivers/system_stm32f4xx.c

@@ -152,6 +152,12 @@ bool isMPUSoftReset(void)
         return false;
 }
 
+void systemClockSetup(uint8_t cpuUnderclock)
+{
+    (void)cpuUnderclock;
+    // This is a stub
+}
+
 void systemInit(void)
 {
     SetSysClock();

src/main/drivers/system_stm32f7xx.c

@@ -146,10 +146,17 @@ bool isMPUSoftReset(void)
         return false;
 }
 
+void systemClockSetup(uint8_t cpuUnderclock)
+{
+    (void)cpuUnderclock;
+    // This is a stub
+}
+
 void systemInit(void)
 {
     checkForBootLoaderRequest();
 
+    //Called by SystemInit from startup_stm32f7xx.s
     //SystemClock_Config();
 
     // Configure NVIC preempt/priority groups

src/main/fc/fc_init.c

@@ -180,6 +180,7 @@ void init(void)
 
     printfSupportInit();
 
+    // Initialize system and CPU clocks to their initial values
     systemInit();
 
     // initialize IO (needed for all IO operations)
@@ -197,9 +198,8 @@ void init(void)
     ensureEEPROMContainsValidData();
     readEEPROM();
 
-#ifdef USE_UNDERCLOCK
+    // Re-initialize system clock to their final values (if necessary)
     systemClockSetup(systemConfig()->cpuUnderclock);
-#endif
     
     i2cSetSpeed(systemConfig()->i2c_speed);