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

# Build script for m4

CT_M4_VERSION=1.4.13

do_companion_tools_m4_get() {

    CT_GetFile "m4-${CT_M4_VERSION}" \

               {ftp,http}://ftp.gnu.org/gnu/m4

}

do_companion_tools_m4_extract() {

    CT_Extract "m4-${CT_M4_VERSION}"

    CT_Patch "m4" "${CT_M4_VERSION}"

}

do_companion_tools_m4_build() {

    CT_DoStep EXTRA "Installing m4"

    mkdir -p "${CT_BUILD_DIR}/build-m4"

    CT_Pushd "${CT_BUILD_DIR}/build-m4"

    CT_DoExecLog CFG \

    "${CT_SRC_DIR}/m4-${CT_M4_VERSION}/configure" \

        --prefix="${CT_BUILDTOOLS_PREFIX_DIR}"

    CT_DoExecLog ALL make

    CT_DoExecLog ALL make install

    CT_Popd

    CT_EndStep

}