rename vopt_onoff to vopt_bool which returns -D<prop>=true|false

This makes it more generic and allows us to use it freely between
cmake and meson build styles.

Manual.md

@@ -830,10 +830,10 @@ package accordingly. Additionally, the following functions are available:
 
   Emits an error and exits if both options are set at the same time.
 
-- *vopt_onoff()* `vopt_onoff <option> <property>`
+- *vopt_bool()* `vopt_bool <option> <property>`
 
-  Outputs `-D<property>=ON` if the option is set, or
-  `-D<property>=OFF` otherwise.
+  Outputs `-D<property>=true` if the option is set, or
+  `-D<property>=false` otherwise.
 
 The following example shows how to change a source package that uses GNU
 configure to enable a new build option to support PNG images:

common/environment/setup/options.sh

@@ -29,12 +29,12 @@ vopt_conflict() {
     fi
 }
 
-vopt_onoff() {
+vopt_bool() {
     local opt="$1" prop="$2"
     if [ "$#" -lt "2" ]; then
-        msg_error "vopt_onoff <build_option> <property>: missing values\n"
+        msg_error "vopt_bool <build_option> <property>: missing values\n"
     elif [ "$#" -gt "2" ]; then
-        msg_error "vopt_onoff $opt: $(($# - 2)) excess parameter(s)\n"
+        msg_error "vopt_bool $opt: $(($# - 2)) excess parameter(s)\n"
     fi
-    vopt_if "$1" "-D${prop}=ON" "-D${prop}=OFF"
+    vopt_if "$1" "-D${prop}=true" "-D${prop}=false"
 }