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

# MicroBlaze specific config options

## depends on EXPERIMENTAL

##

## select ARCH_SUPPORT_ARCH

## select ARCH_FLOAT_SW

## select ARCH_SUPPORTS_BOTH_ENDIAN

## select ARCH_DEFAULT_BE

## select ARCH_SUPPORTS_BOTH_MMU

## select ARCH_DEFAULT_HAS_MMU

##

## help The MicroBlaze architecture, as defined by:

## help 	http://www.xilinx.com/

## help 

## help Upstream projects do not currently provide

## help full support for the microblaze architecture

## help and as such, this is marked as EXPERIMENTAL

## help for CT-NG.

## help 

## help Support is being added for a modern gcc,

## help binutils and gdb along with nptl threading

## help in eglibc for microblaze.