VivadoAccelerator backend updates

[thesps]

A few small things:

• Fix registration of VivadoAcceleratorBackend writer flow. In master branch, the writer flow for that backend is just VivadoBackend's writer flow relabelled, so the extra files for VivadoAcceleratorBackend aren't written out.
• Replace VivadoAcceleratorBackend.make_bitfile method with build(bitfile=True). This is an API change to make that workflow more consistent with the usual build API (rather than requiring one call to build then one call to make_bitfile)
• Add a package method to make a .tar.gz archive with all the necesary files for hardware inference - otherwise the user needs to do some finding and renaming of files themselves

[vloncar]

Can you explain why this hack is necessary? Or, in other words, why isn't the build process blocking so that the package can start only when we're sure the files are there?

[thesps]

I observed cases where the call to os.system returns before the file is written. I guess the process that writes the bitfile is detached.

[vloncar]

What guarantee do you have the the file is fully written when you try to copy it? Perhaps instead of os.system we run subprocess.Popen() because the object it returns has a wait(). Not sure if will help if the main vivado process calls another one and exits.

[thesps]

I got rid of the package function for now. I can't replicate the bitfile writing issue for the moment, it seems to depend on targeting the larger FPGA which takes a long time to build and so getting to the bottom of it will take longer. I intend to add it back as a follow up.

So now the PR does:

• Fix registration of VivadoAcceleratorBackend writer flow. In master branch, the writer flow for that backend is just VivadoBackend's writer flow relabelled, so the extra files for VivadoAcceleratorBackend aren't written out.
• Replace VivadoAcceleratorBackend.make_bitfile method with build(bitfile=True). This is an API change to make that workflow more consistent with the usual build API (rather than requiring one call to build then one call to make_bitfile)