(*) groups distro-dependent files in build/config.<distro>

(*) groups distro-dependent files in build/config.<distro>
(*) renamed .lst files into .pkgs (lst suffix commonly used for other/tmp purposes)
(*) distro-dependent files are (mostly) optional, defaults to planetlab
(*) mkfedora accepts new -f argument : to pass a pkgs file rather than -p .. -g ..
(*) mkfedora supports junk: and precious: in pkgs file
    this is from the former bootcd/prep.sh - to reduce footprint

Notes.
(*) this is untested
(*) mkfedora still supports -x (exclude) on the command line and in pkgs file
    however I suspect this feature is never used
(*) mkfedora is still a separate command - could/should be merged into buld.common
(*) myplc/build.functions could/should be merged into build.common as well