This commit contains several changes:
* it is possible to create manifests for following types of images:
regular image, target SDK and host SDK. To distinguish between these
types of manifests, one has to pass the manifest_type argument to the
contructor or create_manifest() wrapper. The manifest type can have
the following values: image, sdk_host, sdk_target;
* move image_rootfs variable to _create_dummy_initial() since it's used
only here. This function will probably be removed in the future;
* fix a bug in the Dpkg class;
* add INSTALL_ORDER property to Manifest class which contains the
default install order for the packages and will be used Rootfs/Sdk
classes;
