Move version constants into its own compile-unit

Speeds up the build by moving those constants into a separate compile-unit.
Currently, since the version header is auto regenerated on each build,
all modules that include it, also have to be recompiled every time.

lib/FirmwareVersion/src/FirmwareVersion.cpp

@@ -0,0 +1,5 @@
+#include "FirmwareVersion.h"
+#include "generated_version.h"
+
+long FirmwareVersion::BuildEpoch = BUILD_EPOCH;
+const char* FirmwareVersion::VersionString = VERSION_STRING;