Introduce meta-arm-image layer for OpenEmbedded.

meta-arm-image:
- This layer provides a common image shared between Corstone1000 Arm platforms.
- The image is called corstone1000-image.
- Each platform can extend corstone1000-image by including inc files to provide
  custom image settings tailored to the selected machine.

Signed-off-by: Xueliang Zhong <xueliang.zhong@arm.com>