property: Add support for const (read-only) values

There is already support for setting a property as const
(see vtable.hpp and corresponding test cases).
Implement this in the generated code by adding support
for a new yaml flag.

Closes openbmc/openbmc#2028

Tested: Added the const flag to the Software Priority property,
and changing the value via REST and busctl would fail with
"org.freedesktop.DBus.Error.PropertyReadOnly", but the code
update app was able to set its value.

Change-Id: I8534753233080366503fd4f1a0c224c2946e8764
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/example/net/poettering/Calculator.interface.yaml b/example/net/poettering/Calculator.interface.yaml
index af4dd1b..6e44093 100644
--- a/example/net/poettering/Calculator.interface.yaml
+++ b/example/net/poettering/Calculator.interface.yaml
@@ -53,6 +53,8 @@
     - name: Status
       type: enum[self.State]
       default: Success
+      flags:
+        - const
       description: >
         The current state of the Calculator.
     - name: Owner