error: conflicting types for 'rand' unsigned int rand();

The problem could well be because Espruino has its own version of most of the standard library functions like rand (unfortunately required because many of them reference malloc or similar, which pulls in a whole load of extra RAM/Flash usage).

It might be that ST's library actually includes the main standard library, or that it includes its own rand, as you say. But it looks like you can work around that...

As for undefined reference to AES_CBC_Encrypt_Append - it looks to me like you just forgot to add one of the .c files to the Makefile - the one that actually includes the AES_CBC_Encrypt_Append function. If you add that then it might actually compile and work...

PS - the stuff in your post didn't get rendered properly as 'code' because there wasn't a clear line before and after the code markers (```) - I've just changed it for you.