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>

 # Blackfin specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_LE

 ## select ARCH_SUPPORTS_WITH_ARCH

 ## select ARCH_SUPPORTS_WITH_CPU

 ## select ARCH_SUPPORTS_WITH_TUNE

 ## select ARCH_SUPPORTS_WITH_FPU

 ##

 ## help The Blackfin architecture