mkimage-debootstrap.sh: Add -p option to invoke debootstrap with http_proxy.

This patch allows debootstrap to be run with the http_proxy environment
variable. This is principally useful when you're using apt-cacher and
apt-cacher-ng locally.
Merge pull request #2353 from wrouesnel/mkimage-debootstrap-improvements

mkimage-debootstrap.sh: Add -p option to invoke debootstrap with http_proxy