board/firefly_rk3288: add new board

This commit adds basic configuration files for the Firefly RK3288 board.
Both minimal and demo defconfigs are added. The latter enables Qt5 with
the required Mali T76X GPU user space components.

More info about the board:
http://en.t-firefly.com/en/firenow/firefly_rk3288/specifications/



[Peter: use same-as-kernel for kernel headers]
Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>