This is an initial implementation of architecture dependent
contents handling for package lists more or less in the vein
of mkimage-profiles-desktop's one *but* using suffix part to
filter words in or out *not* prefix part to replace it with
a comment marker (thus filtering out lines).

The syntax should be pretty obvious:

  a b@i586 c@x86_64

will get "a b" given ARCH=i586 and "a c" given ARCH=x86_64;
please see doc/archdep.txt for a more elaborate description
and a conversion script.