build-vm ceases to be a target for building only virtual machine images.
Now it can be used to build tarballs designed for installation on real
machines.

This commit is the result of transferring the required functionality from
build-mr (mipsel rootfs) by Ivan Melnikov <iv@altlinux.org>.

NB: mike@ strongly objected to this dilution but gave up eventually;
    the whole kernel/build-vm/tar2fs/pack mess should be split into
    distinct layers busy with their own responsibilities:

    1) a tarball with kernel is done without tar2fs at all
       (and no build-vm bits should be needed either, maybe
       it's worth splitting and renaming as "vm" meaning
       disk image for some armh board is grossly misleading);

    2) a tarball with kernel can be further (multi-)packed
       as, well, (compressed) tarball and a disk image
       (only the latter one should employ build-vm/tar2fs);

    3) compression should be done in pack feature style,
       preferably described once and not duplicated all over
       the profile for every single new kind of its output.

    In the mean time, running into this and moving no further
    starts to hurt more than it could help.