scripts: move backtrace marker to CT_WORK_DIR

Avoid error when commands in scripts/crosstool-NG.sh fail
before CT_BUILD_DIR is set.

So we need to remove the backtrace marker of a potential previous
build. Previously, it was implicitly removed because we did remove
the directory it was in, which is no longer the case.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: remove backtrace marker on start of build]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <20121015094615.GA18673@sig21.net>
Patchwork-Id: 191498

reporter_name="Ioannis E. VENETIS"

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

reporter_comment=""