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.