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.

runtc

@@ -5,7 +5,7 @@ set -- ${cli_runtc:-} "$@"
 while getopts a:gh OPT; do
   case "$OPT" in
     a)
-      arch="$OPTARG"
+      common_arch="$OPTARG"
       ;;
     h)
       printf "Usage: $0 TOOL [TOOL_ARGS]...
@@ -20,6 +20,6 @@ done
 shift "$(($OPTIND - 1))"
 tool="$1"
 shift
-set_common_vars "$arch"
-"${buildroot_out_dir}/host/bin/"*-buildroot-*"${tool}" "$@"
+common_setup
+"${common_buildroot_out_dir}/host/bin/"*-buildroot-*"${tool}" "$@"
 exit "$?"