Allow override of macros in core_version.h (#5268)

Author: SenH

cores/esp8266/core_version.h

@@ -1,5 +1,10 @@
+#ifndef ARDUINO_ESP8266_GIT_VER
 #define ARDUINO_ESP8266_GIT_VER 0x00000000
+#endif
+
+#ifndef ARDUINO_ESP8266_GIT_DESC
 #define ARDUINO_ESP8266_GIT_DESC unspecified
+#endif
 
 // ARDUINO_ESP8266_RELEASE is defined for released versions as a string containing the version name, i.e. "2_3_0_RC1"
 // ARDUINO_ESP8266_RELEASE is used in the core internally. Please use ESP.getCoreVersion() function instead.