Reconfigure Nexys4/Artix7 variant for "stock" functionality

These changes do the following:
1) Eliminate an error during ISE launch regarding the lack of the Pipistrello constraints file
2) Reconfigure the device to use the stock memory map of 32K hub memory, and the full 32K ROM including the full character font and math tables.
3) Constraints are changed for the Nexys4 so that video output is piped directly to the board's VGA connector in a fashion compatible with the Propeller Demo Board pinouts and various video demo apps were tested successfully.