added licensing information for SHA-256 code
added licensing information for SHA-256 code

spark/tinydtls,fvdnabee/tinydtls,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,malvira/tinydtls-code,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,spark/tinydtls,spark/tinydtls,malvira/tinydtls-code,jvermillard/tinydtls,nmacs/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,thecodemaiden/tinydtls-fork,nmacs/tinydtls,OlegHahm/tinydtls,LaKabane/tinydtls,OlegHahm/tinydtls,yunus/TinyDTLS-WebID,nmacs/tinydtls,fvdnabee/tinydtls,OlegHahm/tinydtls,nmacs/tinydtls,LaKabane/tinydtls,jvermillard/tinydtls