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

# Dummy config file to not use a C library *at all*

## depends on BARE_METAL

##

## help Do not use a C library.

## help

## help This is usefull if your project is self-contained, does not rely on

## help an external C library, and provides all the necessary bits.

## help

## help Most probably usefull to bootloaders, as they generally don't depend

## help on the C library.

## help

## help If unsure: do *not* choose that, and use another option in the choice.