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

# Super-H specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORTS_BOTH_ENDIAN

## select ARCH_DEFAULT_LE

##

## help The Super-H architecture, as defined by:

## help     http://www.renesas.com/fmwk.jsp?cnt=superh_family_landing.jsp&fp=/products/mpumcu/superh_family/