Prevent redefine of HSE_VALUE if already user-overridden (stm32duino#1281)

* Prevent redefine of HSE_VALUE if already user-overridden

Commercially sold Blackpill F411CE boards feature a 25MHz crystal, but if a board differs, HSE_VALUE can still be overridden while keeping everything else the same. With defensive programming like this, users of a custom (or differing commercially sold) board can easily use this variant with the same chip but just a slightly different crystal.

Author: maxgerhardt

variants/Generic_F401Cx/variant.h

@@ -110,8 +110,10 @@ extern "C" {
 #define PIN_SERIAL_TX           PA9
 
 #ifdef ARDUINO_BLACKPILL_F401CC
+#ifndef HSE_VALUE
 #define HSE_VALUE               25000000U
 #endif
+#endif
 
 #ifdef __cplusplus
 } // extern "C"

variants/Generic_F411Cx/variant.h

@@ -113,8 +113,10 @@ extern "C" {
 #define PIN_SERIAL_TX           PA9
 
 #ifdef ARDUINO_BLACKPILL_F411CE
+#ifndef HSE_VALUE
 #define HSE_VALUE               25000000U
 #endif
+#endif
 
 #ifdef __cplusplus
 } // extern "C"