CI: Add build-option for custom FPGA design.

The build configuration YAML files describing a custom gateware
configuration will be used by the CI system generate bitstreams for user
custom FPGA designs.

custom-fpga-design/my_custom_fpga_design.yaml

+---
+HSS:
+    type: git
+    link: https://git.beagleboard.org/beaglev-fire/hart-software-services.git
+    branch: develop-beaglev-fire
+    board: bvf
+gateware:
+    type: sources
+    build-args: "M2_OPTION:NONE CAPE_OPTION:GPIOS"
+    unique-design-version: 9.0.2
+