scripts: fix distclean

Do not fail in case either one or both of target/ and
.build/ directories are missing.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Ioannis E. VENETIS"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

reporter_comment=""