image: implement IMAGE_FEATURES

image: implement IMAGE_FEATURES

IMAGE_FEATURES is analagous to DISTRO_FEATURES and MACHINE_FEATURES, for
root filesystem construction. Currently, the only supported features are
any defined package groups, as used by the oe.packagegroup python
module.

Example usage:

    PACKAGE_GROUP_myfeature = "vim iptables"
    IMAGE_FEATURES += "myfeature"

Signed-off-by: Chris Larson <chris_larson@mentor.com>