board target for root Makefile

[nikitos1550]

foo@build-hisi:~/hi3519v101_go$ make
Help:
  - make prepare            - prepare; MUST be done before anything
  - make deploy-app         - build&deploy application onto particular board
  - make deploy-app-control - build&deploy application, then attach serial console onto particular board
  - make control            - attach serial console onto particular board
  - make rootfs.squashfs    - build application and pack it within RootFS image
  - make kernel             - build board kernel
  - make cleanall           - remove all artifacts

Proposal is to add make board target that will build everything except application for setuped board (kernel+rootfs).