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

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"