Simplify stm32 configuration files :
	- stm32f10x.h doesn't need to be board dependant, each board has to define its type in :
		STM32F10X_LD, STM32F10X_LD_VL, STM32F10X_MD, STM32F10X_MD_VL, STM32F10X_HD, STM32F10X_HD_VL,
		STM32F10X_XL, STM32F10X_CL.
	- But system_stm32f10x.c needs to be board dependant, because system clock is set in this file and may depend of external clock.
	- To simplify code that needs HSE_VALUE (when updating system core clock), it's possible to set the HSE value in HSE parameter with cmake if the default HSE value is not used.