Add -Werror to acceptance builds for C and CPP (#4369)

Use platform.local.txt to add -Werror to GCC for the build of all
code.  Any warnings on a submitted patch will cause an error.

Several examples and libraries had warnings/errors (missing returns
on functions, types, etc.).  Clean those up with this commit as well.

libraries/Ethernet/src/utility/socket.cpp

@@ -364,6 +364,7 @@ uint16_t recvfrom(SOCKET s, uint8_t *buf, uint16_t len, uint8_t *addr, uint16_t
  */
 void flush(SOCKET s) {
   // TODO
+  (void) s;
 }
 
 uint16_t igmpsend(SOCKET s, const uint8_t * buf, uint16_t len)