Fix cdc, update example firmware.c

[P-Miranda]

• Fix CDC circuit for send and rcv_ack commands from CPU (at system clock) to ethernet core (at TX_CLK and RX_CLK)
  • now uses LIB macro for Fast to Slow transitions
• Update example_firmware.c
  • Same program for pc-emul and fpga execution
  • Does not require DDR (faster synthesis for fpga)
• Add constraint example for a system with USE_DDR=0