common: simplify set_common_vars, rename to common_setup

Don't pass all arguments explicitly, just use existing vars.

Prefix all common_setup inputs and outputs with common_ to avoid name conflicts.

qemu-trace2txt

@@ -5,12 +5,12 @@ set -- ${cli_qemu_trace2txt:-} "$@"
 while getopts a: OPT; do
   case "$OPT" in
     a)
-      arch="$OPTARG"
+      common_arch="$OPTARG"
       ;;
   esac
 done
-set_common_vars "$arch" false
+common_setup
 ./qemu/scripts/simpletrace.py \
-  "${build_dir}/host-qemu-custom/trace-events-all" \
+  "${common_build_dir}/host-qemu-custom/trace-events-all" \
   "${common_qemu_run_dir}/trace.bin" \
 > "${common_trace_txt_file}"