Added px4fmu_sim branch for software in the loop (SITL) development

I have created a new config under nuttx/config/px4fmu_sim. This builds the nsh console for px4 using the standard linux gcc toolchain. It then creates a virtual romfs and runs the px4 startup script. I wanted to get some feedback from the other developers before going any further. So please let me know what you guys think.

My main question is where should the SITL drivers go. I see that recently all of the code was moved out of nuttx/configs/px4fmu/src to apps/drivers/stm32. Should I create a folder in apps/drivers/sitl? It seems like the nuttx way would be to keep it under configs/px4fmu_sim/src but I can see how that is cumbersome.

Also, if anyone else has interest in helping to develop the SITL drivers, help would be appreciated. Luckily the default nuttx sim arch has a lot of the work done already.