This is a basic rescue mode implementation for d-i.

To include this in an image, add 'rescue-check' to the appropriate file in
pkg-lists. rescue-check will retrieve rescue-mode itself if need be. If
you're building a monolithic image, note that that image type doesn't
include a retriever, so you need to add 'rescue-mode' too.

To use, boot with 'rescue/enable=true', and optionally
'rescue/root=<device>'. I anticipate that bootloader configurations will
eventually include a 'rescue' option with 'rescue/enable=true' set.
