Add raw JTAG support

[colinoflynn]

This adds a single feature from the 'raw jtag' API allowing TMS/TDI/TDO control. This is useful to implement boundary scan mode (which I'm working on in another library called pyjtagbs). Other raw commands could be added, but are harder to use in practice as they require more information about the scan chain sent to the J-Link, most software seems to use this API only.

You should be able to use the code like this:

import pylink
jl = pylink.JLink() #Defaults to JTAG (not SWD)
jl.open() #Open but *do not* connect
jl.jtag_rawrw([0], [0x1F], 5) #JTAG Reset (tms = 1 for 5 bits)
jl.jtag_rawrw([0], [0x02], 4) #JTAG ID read
result = jl.jtag_rawrw([0]*8, [0]*8) #Shift to see what's on scan chain
print(result)
hex_id = hex(struct.unpack("<I", bytes(result[0:4]))[0])
print(hex_id)

If a single item - will just see 4 bytes. You can print the hex_id to see the standard format - testing on a Spartan 6 LX9 shows me 0x3622093 which matches the expected device id.

Note there is no .connect() - you can't connect to the chain as that will start trying to discover devices on it (which may be unsupported devices).

[CLAassistant]

All committers have signed the CLA.

[hkpeprah]

LGTM. Thanks for the changes. Sorry it took me so long to get back to this. Could you add unit tests for the function you introduced? Otherwise, besides the minor comments, LGTM.

[colinoflynn]

No worries on lag - considering the state of the world I hardly think 2 weeks delay for some JTAG raw access is worth apologizing over ;-) I'll figure out the unit test thing - will fix the easy issues first then comment again once tests are there (hadn't looked at how they were working in this repo).