lib/oe/rootfs.py: add new library for rootfs creation

lib/oe/rootfs.py: add new library for rootfs creation

This library will be used to generate the rootfs.

Recommended usage:
 create_rootfs(d, manifest_file)

this will determine the PM backend used, save the shell environment and
will call the appropriate backend implementation (DpkgRootfs(d,
manifest_file).create()).

NOTE: this commit adds Dpkg support.

(From OE-Core rev: 5ad8c21bc7f2213d1287cecf07d00a61d1456ff7)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>