Secure random seed from Spark Cloud
Limit seed to 4 bytes even on future hardware

There are only 5 bytes left in the credentials,
so an attempt on any future hardware that might support
64-bit integers to use sizeof(seed) would read past
the end of the credentials buffer.

For future reference, there would be very little cryptographic risk
in starting the seed at offset 32 and allowing 8 bytes, duplicating
use of the initial message ID and token bytes as part of the seed.
However, there is no need to do so at this time.
random seed from the cloud passed to `random_seed_from_cloud()` function.
this function as weak linkage so that user code can redefine and override this behavior.
missing function.o module in makefile.

I assumed `make test` built the test suite using the libcore-communication-lib.a + headers. "assumptions are bad, mkay?"
removed weak attribute from header - seemed to make it work on linux. I don't fully understand why  - theory is that a weak declaration in the header seems to stop the linker binding the implementation given.
Merge pull request #25 from spark/feature/secure-random-seed

Secure random seed from Spark Cloud