This is a mild generalization of what has been done by hand
to figure out a problem with make-initrd-propagator-0.10-alt1:
stripping anything intrinsically volatile off the build.log.

The filter set isn't perfect, and the proper logging will
involve mkimage tweaks as well (e.g., one generally isn't
interested in instrumental chroots' population that much,
so it should only be verbose at the higher debug levels).