scripts/functions: fix endless loop in debug-shell with IO redirection

CT_DEBUG_INTERACTIVE is disabled when stdin, stdout or
stderr are redirected, but the check is only done at
the start of the build and doesn't catch when individual
build commands use redirection. When stdin is redirected
it will cause the debug shell to exit immediately, causing
and endless loop. Thus, save the stdin/our/err file handles
and restore them before invoking the debug shell.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Message-Id: <20121030102225.GA8303@sig21.net>
Patchwork-Id: 195409

 # This file adds functions to build the C library for bare metal (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_libc_get() {

     :

 }

 do_libc_extract() {

     :

 }

 do_libc_check_config() {

     :

 }

 do_libc_start_files() {

     :

 }

 do_libc() {

     :

 }

 do_libc_finish() {

     :

 }