add missing copyright notice
introduce Linux kernel style build system

The functionality is described as following:

* Makefile.toolchain:
  toolchain-specific configurations; common cflags and ldflags

* platform/build.mk:
  platform-specific configurations; cflags/ldflags for CPU and FPU

* board/*/build.mk:
  board-specific configurations; CHIP model, periperals

* rules.mk:
  the magic of build system