scripts: add option to start an interactive debug shell

Add an option that, when a command fails:
- starts an interactive shell with the failed command's environment
- attempts re-execution of the failed command, continues, or aborts
at user's whim.

Before starting the debug-shell, the backtrace is printed.
When exiting for an abort, the standard error message is printed.

Based on an idea and a patch from: Johannes Stezenbach <js@sig21.net>
http://sourceware.org/ml/crossgcc/2012-09/msg00144.html

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: integrate in the fault handler]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Johannes Stezenbach <js@sig21.net>
Patchwork-Id: 191571
Patchwork-Id: 191668

CT_EXPERIMENTAL=y

CT_LOCAL_TARBALLS_DIR="${HOME}/src"

CT_SAVE_TARBALLS=y

CT_LOG_EXTRA=y

CT_ARCH_CPU="cortex-m3"

CT_ARCH_TUNE="cortex-m3"

CT_ARCH_FLOAT_SW=y

CT_ARCH_arm=y

# CT_ARCH_USE_MMU is not set

CT_ARCH_ARM_MODE_THUMB=y

CT_TARGET_VENDOR="bare_newlib_cortex_m3_nommu"

CT_ARCH_BINFMT_FLAT=y

CT_BINUTILS_V_2_19_1a=y

CT_CC_V_4_4_1=y

CT_CC_LANG_CXX=y

CT_LIBC_VERSION="newlib-1_18_0"

CT_LIBC_NEWLIB_CVS=y

CT_GMP_V_4_3_2=y

CT_MPFR_V_2_4_2=y

CT_PPL_V_0_10_2=y

CT_CLOOG_V_0_15_9=y