Add JTAG support

[andreondra]

This will save a lot of time during debugging. We can use some ready-to-use core:

• https://opencores.org/projects/jtag
• https://www.xilinx.com/products/intellectual-property/bscan-to-jtag-converter.html

This will need further research. JTAG allows not only to simply check registers' values but also to debug C code using GDB, which can be very handy.

[andreondra]

Apart from JTAG, Xilinx integrated debug cores and logic analyzers may be used: https://docs.xilinx.com/r/en-US/ug908-vivado-programming-debugging/Customizing-and-Generating-the-Debug-Cores