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_LOCAL_TARBALLS_DIR="${HOME}/src"

CT_SAVE_TARBALLS=y

CT_LOG_EXTRA=y

CT_ARCH_64=y

CT_ARCH_x86=y

CT_KERNEL_linux=y

CT_KERNEL_V_2_6_33_20=y

CT_BINUTILS_V_2_20_1a=y

CT_BINUTILS_FOR_TARGET=y

CT_CC_V_4_4_3=y

CT_CC_LANG_FORTRAN=y

CT_CC_LANG_CXX=y

CT_CC_LANG_JAVA=y

CT_LIBC_uClibc=y

CT_LIBC_UCLIBC_V_0_9_30_2=y

CT_LIBC_UCLIBC_CONFIG_FILE="${CT_LIB_DIR}/samples/${CT_TARGET}/${CT_LIBC}-${CT_LIBC_VERSION}.config"

CT_DEBUG_dmalloc=y

CT_DEBUG_duma=y

CT_DEBUG_gdb=y

CT_GDB_NATIVE=y

CT_DEBUG_ltrace=y

CT_DEBUG_strace=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