compliance: Start work on #27

- Documented a work flow for the compliance tests in tests/compliance/README.md

- A single (not very good) test for sha2sum0 is there as an example.

- Passes on Spike and SAIL (for what it's worth right now).

- Next steps are to generate the tests from the kat generator.

 On branch dev/next-release
 Your branch is up-to-date with 'origin/dev/next-release'.

 Changes to be committed:
	modified:   .gitmodules
	modified:   bin/conf.sh
	new file:   extern/riscv-compliance
	new file:   tests/compliance/Makefile
	new file:   tests/compliance/README.md
	new file:   tests/compliance/riscv-compliance.patch

 Changes not staged for commit:
	modified:   extern/riscv-compliance (modified content)
	modified:   extern/riscv-gnu-toolchain (modified content)
	modified:   extern/riscv-isa-sim (modified content)
	modified:   extern/sail-riscv (modified content, untracked content)