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

 reporter_name="Anthony Foiani <anthony.foiani@gmail.com>"

 reporter_url="http://sourceware.org/ml/crossgcc/2010-09/msg00100.html"

 reporter_comment="This is a sample config file for Freescale e500v2 processors (e.g., MPC8548,

 MPC8572). It uses eglibc (for e500/SPE patches) and a recent gcc (4.6.0,

 for e500v2 DPFP support) and will generate appropriate dual-precision

 floating point instructions by default.

 Note: If building a Linux kernel with this toolchain, you will want to make

 sure -mno-spe AND -mspe=no are passed to gcc to prevent SPE ABI/instructions

 from getting into the kernel (which is currently unsupported). At this time,

 the kernel build system properly passes those two options, but older kernels

 were only passing -mno-spe by default."