Rename mainapp to px4.

Documentation/px4_hil/UserGuide.md

Tools/decode_backtrace.py

Tools/sitl_multiple_run.sh

Tools/sitl_run.sh

cmake/test/px4_simple_app_correct.txt

posix-configs/SITL/README.md

posix-configs/bebop/px4.config

posix-configs/rpi/px4.config

src/firmware/posix/CMakeLists.txt

src/firmware/qurt/CMakeLists.txt

src/platforms/posix/main.cpp

src/platforms/qurt/px4_layer/main.cpp