Linux: Added preliminary work queue support

src/platforms/linux/px4_layer/module.mk

src/platforms/linux/px4_layer/px4_linux_impl.cpp

src/platforms/linux/px4_layer/work_cancel.c

src/platforms/linux/px4_layer/work_queue.c

src/platforms/linux/px4_layer/work_thread.c

src/platforms/px4_config.h

src/platforms/px4_defines.h

src/platforms/px4_tasks.h

src/platforms/px4_workqueue.h