vopt_bool: use option as default property

Keep it in-line with vopt_with and vopt_enable.

common/environment/setup/options.sh

@@ -30,10 +30,8 @@ vopt_conflict() {
 }
 
 vopt_bool() {
-    local opt="$1" prop="$2"
-    if [ "$#" -lt "2" ]; then
-        msg_error "vopt_bool <build_option> <property>: missing values\n"
-    elif [ "$#" -gt "2" ]; then
+    local opt="$1" prop="${2:-$1}"
+    if [ "$#" -gt "2" ]; then
         msg_error "vopt_bool $opt: $(($# - 2)) excess parameter(s)\n"
     fi
     vopt_if "$1" "-D${prop}=true" "-D${prop}=false"