Add a few tweaks and fixes to contrib/mkimage-debian.sh

Add simple workaround for #1755 to contrib/mkimage-debian.sh

Add simple echo patch in contrib/mkimage-debian.sh to prevent init scripts from running during apt-get installs/updates

Add `apt-get clean` to mkimage-debian.sh for slightly smaller images

Add more small apt tweaks to mkimage-debian.sh thanks to @jpetazzo and @spahl