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

 # Compute microblaze specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     # CFLAGS

     case "${CT_ARCH_FLOAT_HW},${CT_ARCH_FLOAT_SW}" in

         y,) CT_ARCH_FLOAT_CFLAG="-mhard-float" ;;

         ,y) CT_ARCH_FLOAT_CFLAG="-msoft-float" ;;

     esac

 }