samples: 96b_argonkey: Remove board.h
samples: 96b_argonkey: Remove board.h

We don't need board.h, but by including it we would end up picking up
stdio.h from somewhere which we need for sprintf.  So just included
stdio.h and drop board.h

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

explora26/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr