[WIP][µTVM] Add OpenOCD Low-Level Device (RISC-V Support)

[weberlo]

This PR adds a low-level device backend to µTVM (initial implementation here) that enables the use of any device which features a JTAG interface. In order to do so, OpenOCD is used to mediate a connection between µTVM and the device. In particular, OpenOCD exposes a TCP socket, which µTVM connects to and uses to send Tcl commands to the device.

There are still some unimplemented features:

• Instructions for how to set up OpenOCD
• A cleaner binutil interface
• A better method for configuring the low-level device

CC @Mutinifni @u99127 @liangfu @ajtulloch @tqchen

[tqchen]

See also comments. The main improvement that I think we could make here is to avoid repetitive creation of temporary std::string objects during command construction and receive, by making buffers as state of the tcl_socket .

For example, the API below makes cmd_stream and reply as part of the state. Although it won't eliminate all of creation deletion of std::string, it will somewhat reduces the cost.

socket_.cmd_stream() << "array unset output";
socket_.SendCommand();
res = socket_.last_reply();

[tqchen]

Thanks @weberlo this PR is now merged