Restructure Tcl script entrypoint.

vivado.tcl is now the entrypoint for the Vivado Tcl scripts and will
automatically source all the other required scripts.

A command line argument parser was written and replaces the previous system of
using environment variables to pass values into the scripts. The VSRCSVIVADOTCL
environment variable has been replaced with a -F command line option, and the
file format has changed from a Tcl script to a simple newline-delimited list of
files.