[J-core] j2 repo up on github.

[Rob Landley]

The j2 build for turtle is live on github:

  https://github.com/j-core/jcore-soc

It uses submodules, so you'll need to "git submodule init; git submodule update"
because the top level repo pulls in 3 subrepos. (There were two previous tarball
snapshots, but this is an actual repo we can update going forward.)

To use that you'll need to install the xilinx vhdl toolchain (there's old but
still working instructions for webice/s6 on j-core.org), and the bootloader
build needs an sh2 bare metal elf toolchain to build the bootloader (see
https://lists.j-core.org/pipermail/j-core/2019-October/000867.html), and it uses
the device tree compiler (ala https://git.kernel.org/pub/scm/utils/dtc/dtc.git)
to embed a dtb it can hand over to Linux.

I'm working on updating the README...

Thanks to spacechain for sponsoring the engineer time to finally get this
properly packaged up:

https://www.prnewswire.com/news-releases/spacechain-foundation-invests-in-core-semiconductor-to-produce-open-hardware-platform-for-direct-satellite-to-devices-communication-301061761.html

Next up, we need to get the FPGA boards this builds bitstreams for up on
crowdsupply. :)

Rob

P.S. coresemi is basically the successor to SEI. New company, mostly the same
engineers.