remove perl dependencies from extlinux and thus from stage1

closes #122

the replacement C program and the needed research were mostly
done by @prazzb - thanks!
patchutils are written in perl
busybox tar: support xz compression with J flag
busybox: make tar cJf work for real

the previous patch only added the flag, but its usage leads to
uncompressed tarballs, which disguise themselves as compressed.
stage1: remove xz, busybox supports extracting xz already
core/pkg: add xz
remove xz dependency from packages using xz tarballs
utils/find-pkgs-with-dep.pl: use S instead of R