x86: dts: use BOARD Kconfig symbol not SOC for arduino_101
x86: dts: use BOARD Kconfig symbol not SOC for arduino_101

As the device tree is board specific we should be using the board
Kconfig variable to decide which .dts to use and not the SoC one for
arduino_101.

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

galak/zephyr,fbsder/zephyr,punitvara/zephyr,galak/zephyr,explora26/zephyr,rsalveti/zephyr,fbsder/zephyr,punitvara/zephyr,Vudentz/zephyr,mbolivar/zephyr,kraj/zephyr,runchip/zephyr-cc3220,kraj/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,runchip/zephyr-cc3220,nashif/zephyr,explora26/zephyr,punitvara/zephyr,nashif/zephyr,fbsder/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,kraj/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,Vudentz/zephyr,rsalveti/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,ldts/zephyr,fbsder/zephyr,zephyriot/zephyr,nashif/zephyr,zephyriot/zephyr,finikorg/zephyr,rsalveti/zephyr,fbsder/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,explora26/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,kraj/zephyr,punitvara/zephyr,ldts/zephyr