SystemCoreClock is not set correctly

[AlanFord]

system_stm32f1xx.c initially sets SystemCoreClock to a value of 16000000, while on startup the STM32F1 series defaults to using the 8 MHz HSI clock. This leads to incorrect operation of peripherals whose configuration depends on SystemCoreClock. Calling SystemCoreClockUpdate() during program initialization corrects the value stored in SystemCoreClock.