TT#28113 Rename instead of overwriting the target config file

Files that are being executed cannot be modified in place as that
returns EBUSY. But their dentry can be replaced with a rename(2),
which is atomic and does not touch the original inode. This is the
standard procedure to replace running executables in Unix.

So, we just replace the cat(1) with a mv(1), and stop quiescing its
stderr so that we get proper errors reported.

Change-Id: If15ea1cfa749a6140ff4022200c7fc730c76aa3a

helper/build_config

@@ -82,10 +82,9 @@ tt_tmp_output_file="$(mktemp ngcpcfg."${tt_tmp_output_file_base}".XXXXXXXXXX)"
 
 log_debug "Output file ${output_file} based on ${input_file}"
 log_debug "Executing: $TT_WRAPPER ${input_file} > ${tt_tmp_output_file}"
-log_debug "      and: cat ${tt_tmp_output_file} > ${output_file}"
+log_debug "      and: mv ${tt_tmp_output_file} ${output_file}"
 if "$TT_WRAPPER" "${input_file}" > "${tt_tmp_output_file}" 2>/dev/null &&
-   cat "${tt_tmp_output_file}" > "${output_file}" 2>/dev/null ; then
-  rm -f "${tt_tmp_output_file}"
+   mv "${tt_tmp_output_file}" "${output_file}" ; then
   log_info "Generating ${output_file}: OK"
   RC=0
 else