[IMAGE][CLEA-OS] Add basic support to Clea OS image

Minimal image that inherits from core-image. The .inc file is used to extend the image feature with out custom class clea-os-mage.bbclass. Today this class is empty but will be populated in the nearly future. A generic include file has been used, instead of applying the inner code directly inside the bb file, to give the possibility to create another image recipe from scratch and in the same time inherit the clea-os image feature.

Note: current bootscript does not load and use the ramfs.img file but this last is present inside the wic image.

Note: the produced ramfs.img is compatible only with ARM architecture.

Signed-off-by: Davide Cardillo davide.cardillo@seco.com