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>

 # Compute AVR32-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     CT_TARGET_SYS=none

 }