I am also experiencing this issue of no connectivity when trying to access the PX4 nsh over UART on the Discovery board. I'm on the latest stable branch, no build or programming errors.
I verified my wiring by flashing stock NuttX to the board, and was able to access nsh over both USB and UART2 w/ FTDI cable no problem. Switching back to the PX4 discovery bootloader + px4-stm32f4discovery_default build, I get nothing on either port.
Any assistance would be greatly appreciated.
edit: I am on Ubuntu 14.04 using arm-none-eabi-gcc version 4.8.4