portable: include base and extension images in log fields

portable: include base and extension images in log fields

When a portable service uses extensions, we use the 'main' image name
(the one where the unit was found in) as PORTABLE=. It is useful to
also list all the images actually used at runtime, as they might
contain libraries and so on.

Use PORTABLE_ROOT= for the image/directory that is used as RootImage=
or RootDirectory=, and PORTABLE_EXTENSION= for the image/directory that
is used as ExtensionImages= or ExtensionDirectories=.

Note that these new fields are only added if extensions are used,
there's no change for single-DDI portables.

Example with a base and two extensions, with the unit coming from the
first extension:

[Service]
RootImage=/home/bluca/git/systemd/base.raw
Environment=PORTABLE=app0.raw
BindReadOnlyPaths=/etc/os-release:/run/host/os-release
LogExtraFields=PORTABLE=app0.raw
LogExtraFields=PORTABLE_ROOT=base.raw

ExtensionImages=/home/bluca/git/systemd/app0.raw
LogExtraFields=PORTABLE_EXTENSION=app0.raw

ExtensionImages=/home/bluca/git/systemd/app1.raw
LogExtraFields=PORTABLE_EXTENSION=app1.raw