Flashing nuttx on pixhawk 2

Hello everyone,
can someone help me with flashing nuttx on pixhawk. As nuttx does not have pixhawk board i added pixhawk to nuttx configs but getting error in building nuttx

Why don’t you use MiossionPlanner to Flash the FW like everybody else ?

And If you want a bare metal RealTime OS, Why Nuttx? Why not ChibiOS ?