add new board config for fmu-v3

Images/px4fmu-v3.prototype

cmake/configs/nuttx_px4fmu-v3_default.cmake

nuttx-configs/px4fmu-v3/include/board.h

nuttx-configs/px4fmu-v3/include/nsh_romfsimg.h

nuttx-configs/px4fmu-v3/nsh/Make.defs

nuttx-configs/px4fmu-v3/nsh/appconfig

nuttx-configs/px4fmu-v3/nsh/defconfig

nuttx-configs/px4fmu-v3/nsh/setenv.sh

nuttx-configs/px4fmu-v3/scripts/ld.script

nuttx-configs/px4fmu-v3/src/Makefile

nuttx-configs/px4fmu-v3/src/empty.c

src/drivers/boards/px4fmu-v3/board_config.h