Allow F_CPU redefinition

To avoid any issue with `F_CPU` value, it is defined by default to
`SystemCoreClock` value which is updated automatically after each
clock configuration update.

Some libraries use `F_CPU` at build time for conditional purpose
(ex to #612).
This commit allow to redefine it at build time using `build_opt.h`
or `hal_conf_extra.h`:
https://github.com/stm32duino/wiki/wiki/Custom-definitions
then it will be possible to define it as a constant.

Important note:
**User have to ensure to set it to the proper value.**

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/stm32/stm32_def.h

@@ -20,7 +20,6 @@
                                         |(STM32_CORE_VERSION_PATCH << 8U )\
                                         |(STM32_CORE_VERSION_EXTRA))
 
-#define F_CPU SystemCoreClock
 #define USE_HAL_DRIVER
 
 #ifdef STM32F0xx
@@ -53,6 +52,10 @@
 #error "STM32YYxx chip series is not defined in boards.txt."
 #endif
 
+#ifndef F_CPU
+#define F_CPU SystemCoreClock
+#endif
+
 // Here define some compatibility
 #ifndef CAN1
 #define CAN1 CAN