steer-control: move out steering logic to plugins

src/Makefile

src/cntlr.c

src/cntlr.h

src/cntlr_ubus.c

src/config.c

src/config.h

src/plugins/steer/rcpi/Makefile

src/plugins/steer/rcpi/rcpi.c

src/steer.h

src/steer_module.c

src/steer_module.h