Moved custom config files into it's own directory.

This way the all stay grouped together and the 'real' installer files
are the only ones in the root.
Adjust building the .img.xz file so the configs directory is also part of it.
Handle the addition of the configs directory during install.

If the configs directory is present, back them up before rewriting the
/boot/ partition. Later on, copy those files back in.
Cat, not copy the config files into place.
Make invocation of locale-gen safer.