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.

This change is done to reduce ambiguity in some cases;
the previous intention has been to ease navigation when
staying in a particular directory, now it's been changed
in favour of convenient toplevel `git grep' in fact.

Both variants have their pros and cons, I just find myself
leaning to this one by now hence the commit.  Feel free to
provide constructive criticism :)

Some path-related bitrot has also been fixed while at that.

Yes, mkimage-profiles is now able to build VM disk images.
So far the support is pretty basic:

- a single hard drive image with a single partition/FS
- only stock root password is configurable
- LILO is hardwired as a bootloader

The resulting images tend to boot under qemu/kvm though.

Please see doc/vm.txt for the warning regarding additional
privileges and setup required.  This was started back in
February but I still hoped to avoid sudo/privileged helper
(and libguestfs is almost as undistributable as can be)...

Thanks:

- http://blog.quinthar.com/2008/07/building-1gb-bootable-qemu-image-using.html
- Alexey Morarash who reworked that as https://github.com/tuxofil/linsygen
- led@, legion@, vitty@, aen@ for providing advice and inspiration