Wsign-compare: Fixed all sign compare warnings in src

This commit addresses all warnings emitted from -Wunused-function
in the C++ src.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/xbee/xbee.cxx

@@ -94,7 +94,7 @@ bool XBee::commandMode(std::string cmdChars, int guardTimeMS)
 
 string XBee::stringCR2LF(string str)
 {
-  for (int i=0; i<str.size(); i++)
+  for (size_t i=0; i<str.size(); i++)
     if (str[i] == '\r')
       str[i] = '\n';