Make charmap-converting a non-UTF8 string non-fatal

include/asm/util.h

@@ -13,6 +13,9 @@
 
 uint32_t calchash(const char *s);
 char const *print(int c);
+/*
+ * @return The number of bytes read, or 0 if invalid data was found
+ */
 size_t readUTF8Char(uint8_t *dest, char const *src);
 
 #endif /* RGBDS_UTIL_H */