Introduce and output id, which is the checksum over a full pipeline,
including all stages and the assembler. The id of a pipeline did not
include assemblers before. To be less confusing, rename the existing id
to "tree id".
Take this as an argumnet to __init__ in the same way that `base`
is.
This avoids us having to deal with the case of someone setting a
stage before the build, which does not work as the stage id will
be wrong.
Signed-off-by: Tom Gundersen <teg@jklm.no>