meta/classes/rootfs_rpm.bbclass: re implement base on rpm5
Re implement the rootfs generation using rpm5. This also gets rid of the
need for yum, and handles all dep resolving internal to the script itself.
The new file scripts/rootfs_rpm-extract-postinst.awk comes from the original
yum integration work. It has been unchanged, but since yum is no longer used
we needed to move the script somewhere else.
