Split up --base-image into --base-tree and --overlay

Split up --base-image into --base-tree and --overlay

--base-tree indicates a base image. It differs from --skeleton-tree
in that use of --base-tree indicates that we've already installed a
distribution and should only install extra packages, not install the
distribution from scratch.

If --overlay is not specified, we just copy all the specified base trees
to the root directory before doing anything else (even before copying the
skeleton trees) and after that we operate as usual.

If --overlay is specified, instead of copying all the base trees to
the root directory, we set up an overlayfs mount with all the base
trees as lowerdirs. After that we operate as usual. The effect is that
all our usual steps will operate on a full view of the image, but the
output will only contain the additions we made on top of the specified
base trees.