This allows allows shortcutting measurements of the specified files and
use the information from /sys/ instead.
This is not too useful on its own given that "systemd-measure status"
already exists which displays the current, relevant PCR values. The main
difference is how "complete" the information is. "status" will detect if
the measurements make any sense, and show more than PCR 11. "calculate
--current" otoh only reads PCR 11 and uses that, and that's really it.
This is mainly preparation for later work to add PCR signing to the
tool, where usually it makes most sense to sign prepared kernel images,
but for testing it's really useful to shortcut signing to the current
PCR values instead
prefix_roota() is something we should stop using. It is bad for three
reasons:
1. As it names suggests it's supposed to be used when working relative
to some root directory, but given it doesn't follow symlinks (and
instead just stupidly joins paths) it is not a good choice for that.
2. More often than not it is currently used with inputs under control of
the user, and that is icky given it typically allocates memory on the
stack.
3. It's a redundant interface, where chase_symlinks() and path_join()
already exist as better, safer interfaces.
Hence, let's start moving things from prefix_roota() to path_join() for
the cases where that's appropriate.
