This patch decouples GoferMountConf to two layers to allow us to
configure all combinations of a gofer mount in a succinct way:

- Upper layer config: none, memory, self, anon. The upper layer
  is always tmpfs. It describes the backend for tmpfs.
- Lower layer config: none, lisafs. It describes the backend for
  the filesystem which actually holds the image contents.

The old SelfTmpfs will be represented as "upper=self,lower=none",
MemoryOverlay will be "upper=memory,lower=lisafs", SelfOverlay
will be "upper=self,lower=lisafs", and so on. Thanks to @ayushr2
for the suggestion on how to better decouple this.

This is a preparation for adding the EROFS rootfs support. There
is no functional change intended.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>

This patch refactors the overlay medium from string to a new type
OverlayMedium. Now all overlay medium related methods, such as
validation and extracting anon directory, are implemented on this type.

This is a preparation for adding the EROFS rootfs support. There
is no functional change intended.

Co-authored-by: Ayush Ranjan <ayushranjan@google.com>
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>

This patch adds the EROFS rootfs support. No gofer process will be
created for the container, when there is no need to pass through host
files into a container via the gofer process. Annotations for rootfs
are also introduced to provide extra information, including the mount
source, mount type and overlay config. Additionally, busybox-static
is added to the default image and will be used to build the EROFS
rootfs images during the test.

Updates google#8956

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>