Clean up build directory

There are transient artifacts left behind from previous
builds that can interfere with the current build.

This change deletes the contents of deb_dist to make
sure the packages are fresh with each build.

Change-Id: I127261739929f6b4ac44dc35342ce9580d9301c1
Signed-off-by: beierlm <mark.beierl@canonical.com>