Code review comment for lp:~mterry/ubiquity/oem-config-merge

OK, so this is a first pass at merging oem-config into ubiquity.

Notes:
 * As far as possible, I wanted to avoid two copies of essentially the same code. Experience shows that one will always bitrot, no matter how close it is in the file tree to its sibling. So where necessary, I used an environment variable to indicate whether we were in 'oem-config mode' or not.

 * The oem-config binary is just a symlink to ubiquity. Ubiquity sees what name it's being run as, and sets a variable accordingly.

 * I didn't try to keep OEM_CONFIG_* environment variables. Where used, I changed them to UBIQUITY_* variables.

 * oem-config had some basic support for preseeding a list of pages to show. This seemed not a terrible idea, and a baby step towards drop-in pages, so I converted ubiquity to use the same system. Especially since oem-config and ubiquity would have different page lists. I only tested ubiquity and oem-config's default page lists. Supporting all iterations of pages would be difficult and not recommended. But the start of such plug-and-play support is there.

 * I ported the debconf frontend for oem-config, but I'm not sure I did it right. I couldn't quite get it to run even with old oem-config. I need some guidance on how this is supposed to behave.

 * There are some d-i packages that hardcode '/target'. Where necessary, ubiquity makes and saves an oem-config-friendly version. Ideally we'd eventually make patches for those packages to accept an argument of what the target is.

I'm sure there's more to say about this, but I'll take questions now. :)