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

# MIPS specific config options

## select ARCH_SUPPORTS_32

## select ARCH_SUPPORTS_64 if EXPERIMENTAL

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORTS_BOTH_ENDIAN

## select ARCH_DEFAULT_BE

## select ARCH_SUPPORTS_WITH_ARCH

## select ARCH_SUPPORTS_WITH_TUNE

## select ARCH_SUPPORTS_WITH_FLOAT

##

## help The MIPS architecture, as defined by:

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