[ion] Add getter method for PCB version in string form

ion/include/ion.h

@@ -34,6 +34,7 @@ const char * serialNumber();
 const char * softwareVersion();
 const char * patchLevel();
 const char * fccId();
+const char * pcbVersion();
 
 // CRC32 : non xor-ed, non reversed, direct, polynomial 4C11DB7
 uint32_t crc32Word(const uint32_t * data, size_t length); // Only accepts whole 32bit values