Add RISC-V support.

This supports both 0.11 and 0.13 versions of the debug spec.

Support for `-rtos riscv` will come in a separate commit since it was
easy to separate out, and is likely to be more controversial.

Flash support for the SiFive boards will also come in a later commit.

Change-Id: I1d38fe669c2041b4e21a5c54a091594aac3e2190
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4578
Tested-by: jenkins
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/helper/log.h

@@ -149,6 +149,8 @@ extern int debug_level;
  */
 #define ERROR_FAIL						(-4)
 #define ERROR_WAIT						(-5)
+/* ERROR_TIMEOUT is already taken by winerror.h. */
+#define ERROR_TIMEOUT_REACHED			(-6)
 
 
 #endif /* OPENOCD_HELPER_LOG_H */