Here we go with postprocessing priorities along the way
as ISO hybridization has to occur before implanting
final MD5 sum (which must happen earlier than e.g. some
external MD5SUM file generation).

Unfortunately proper dependencies aren't applicable here
(though I'd like to be proved wrong on this one).

Please note that this needs propagator > 20101130-alt9
for automatic mode to work (has also been worked around
in gfxboot case with design-bootloader-source-6.0-alt1).

Thanks rom_as@ for asking about the hybrid image status
and helping out with testing.

The features might get copy-pasted (or even copied-and-pruned)
when initialized; there's an unneccessary duplication of the
function name in the line adding it to FEATURES list, thus
prone to being forgotten and causing some havoc later on.

It was wrong in the first place but tackling this with some
double-colon rules ran into terminality issues, and further
tortures were considered unneccessary.

The current solution isn't perfect (no completely transparent
function name registration upon corresponding target being called)
but at least it is an improvement...

This is a base for "media check" to become available:
using this feature will implant a checksum into the image
so that it can be verified during install.

Also added a test/demo distro/live-isomd5sum target.
For real distros an alterator module is probably due.