scripts/functions: return a proper error code in CT_DoExecLog

Since we added the debug-shell feature, CT_DoExecLog no longer
returns the error code of the command, but always return 0.

This breaks the download mechanism, which relies on CT_DoExecLog
to fail _on_purpose_ to detect that the ressource was not found
at the specified URL.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # 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() {

     :

 }