scripts/extract_dts_includes.py: Allow for includes in common yaml files
scripts/extract_dts_includes.py: Allow for includes in common yaml files

This patch adds the capability to include yaml files in other yaml
files that reside in dts/common/yaml.

Fixes #1149

Signed-off-by: Andy Gross <4ba87047449f5700e8758e3b1af36e94376c2fff@linaro.org>

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