core: distclean is a noconfig target

core: distclean is a noconfig target

When the .config contains invalid configuration *(e.g. two providers for
a same virtual package), it is not possible to run "make distclean"
because the .config file is sourced and packages interpreted in this
case.

Add distclean to the noconfig list, so that we can run it in all cases.

However, this meand that DL_DIR is no longer set, and thus the default
download location never removed. We fix that by always removing the
download location, so that if it is the one configured we still remove
it (no change) and if it is not the one configured, we remove an
non-existing location and leave the user's location intact (no change
either).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>