Gateware repostiory re-organization

The purpose of these changes is to reduce the number of repositories used to build a bitstream from 4 to just 2 (this repo and the upstream HSS). The bitstream-builder functionalities are moved to the gateware repository where a single scripts (build-bitstrream.py) will build the HSS and gateware and generate one or more bitstreams.

Changes:

• Move the MSS_Configuration into the source directory. This removes the need to access an external repository for retrieving the MSS configuration and keeping it in synch with the gateware project.
• Move the gateware scripts into the source directory.
• Merge in the bitstream-builder repository scripts into this repo.
• Add the custom-fpga-design directory containing a gateware build options YAML file for customizing gateware design(s). The bitstream for the configuration(s) in this directory will be generated by the BeagleV-Fire CI on commits to a forked repo on git.beagleborard.org.