Fix contrib/mkimage/debootstrap for Lucid

Later versions of `apt` are smart enough to just create this directory if it's missing, but Lucid balks (just like `gnupg` and `gpgv` aren't in the Required set so don't come in a minbase -- good ol' Lucid).

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Merge pull request #7545 from tianon/mkimage-lucid-fix

Fix contrib/mkimage/debootstrap for Lucid