crypto: jitterentropy - avoid compiler warnings

The core of the Jitter RNG is intended to be compiled with -O0. To
ensure that the Jitter RNG can be compiled on all architectures,
separate out the RNG core into a stand-alone C file that can be compiled
with -O0 which does not depend on any kernel include file.

As no kernel includes can be used in the C file implementing the core
RNG, any dependencies on kernel code must be extracted.

A second file provides the link to the kernel and the kernel crypto API
that can be compiled with the regular compile options of the kernel.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>