add cli option to dump config settings

[James Hunt]

The upgrader tool that works with the image previously downloaded by system-image-cli needs to know where to find said download files.

Currently, it could look in /etc/system-image/client.ini, but now that system-image is growing the ability to split its configuration across multiple files (bug 1373467), that strategy will not be sufficient.

The 2 most obvious approaches would seem to be:

1) Allow the upgrader tool to import a system-image python module that allows it to query these config settings.

2) Add an option to system-image-cli that allows the config settings to be dumped (à la 'system-image-cli --show-settings'). This would allow the upgrader to run something like the following to query the download location:

$ system-image-cli --get-config-option updater.cache_partition
/userdata/cache

[Barry Warsaw (barry)]

mvo: I've committed a fix to the git repo for this. Let me know if it does the trick for snappy and if there's anything else we need to do to unfork the snappy version of si. I can release this fairly quickly if it does.