Add --synthesis-template arg to parse.py

[ballifatih]

I like the fact that parse.py has create_yosys_script function, that takes a yosys template synthesis script and produces the final script by populating READ_FILES, TOP_MODULE etc. variables conveniently.

In my use case, I needed to make a quite a few modifications in the yosys script so I was using --synthesis-file to provide the script. However, since this argument does not pass through create_yosys_script function, parse.py does not automatically handle source files, module names etc. On the other hand, as far as I understand, there is not an elegant way to access environment inside a yosys script, and therefore one ends up hard coding the source file paths into the yosys script. This was problematic in my use case, because

1. I invoke CocoAlma flow outside its own directory, and
2. My source files are spread around few different directories, and hard coded paths would not work for other folks who would like to run the same flow.

I think it would be great to have a arg to pass template yosys script (instead of the final yosys script). This PR brings this change.

It could also be possible to remove this --synthesis-template and instead define a new arg --path-synthesis-file that changes how --synthesis-file is interpreted.

cc: @vedadux @vogelpi