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>

reporter_name="Kevin Cernekee"

reporter_url="http://sourceware.org/ml/crossgcc/2011-09/msg00037.html"

reporter_comment="MIPS SDE-compatible toolchain"