systemlib delete unused board_serial

src/drivers/px4fmu/fmu.cpp

src/modules/systemlib/CMakeLists.txt

src/modules/systemlib/board_serial.c

src/modules/systemlib/board_serial.h

src/modules/uavcan/uavcan_main.cpp

src/modules/uavcan/uavcan_servers.cpp

src/modules/uavcanesc/uavcanesc_main.cpp

src/modules/uavcannode/uavcannode_main.cpp