Allow robotics cape to be included in gateware design with any name.

Support PWM and H bridge GPIO control for the Robotics cape.

- Optimize MSS GPIO_2 usage.
- GPIOS build option now uses GPIOs for all cape pins except I2C on
  P9_PIN19 and P9_PIN20 which are hard-wired to MSS I2C.
- Move user LEDs control to MSS GPIO_2.

The TOP_LEVEL_NAME build argument allows choosing the name of the
gateware's top level name. This can be used to identify different
variants of the gateware design.

- Add missing cape signals.
- Adjust M.2 and base design pin constraints to match new cape pin
  assignments
- Move immutable cape signasl pin constraints to base design's pin
  constraints file.

Let I/O pads propagate to the top level of the design and rename as
required.