[flight_plan] allow variable declaration from flight plan

- can be used localy in the flight plan to avoid adding new variables in
  the base code when it can be avoided
- with appropriate attributes, will generate a settings accessible from
  the GCS

conf/flight_plans/flight_plan.dtd

@@ -1,6 +1,6 @@
 <!-- Paparazzi flight plan DTD -->
 
-<!ELEMENT flight_plan (header?,waypoints,sectors?,includes?,exceptions?,blocks)>
+<!ELEMENT flight_plan (header?,waypoints,sectors?,variables?,includes?,exceptions?,blocks)>
 
 <!ELEMENT procedure (param*,header?,waypoints?,sectors?,exceptions?,blocks?)>
 
@@ -14,6 +14,9 @@
 <!ELEMENT corner EMPTY>
 <!ELEMENT kml EMPTY>
 
+<!ELEMENT variables (variable*)>
+<!ELEMENT variable EMPTY>
+
 <!ELEMENT includes (include*)>
 
 <!ELEMENT exceptions (exception*)>
@@ -86,6 +89,22 @@ name CDATA #REQUIRED>
 <!ATTLIST kml
 file CDATA #REQUIRED>
 
+<!ATTLIST variables>
+
+<!ATTLIST variable
+var CDATA #REQUIRED
+type CDATA #IMPLIED
+init CDATA #IMPLIED
+shortname CDATA #IMPLIED
+min CDATA #IMPLIED
+max CDATA #IMPLIED
+step CDATA #IMPLIED
+unit CDATA #IMPLIED
+alt_unit CDATA #IMPLIED
+alt_unit_coef CDATA #IMPLIED
+values CDATA #IMPLIED>
+
+
 <!ATTLIST blocks>
 
 <!ATTLIST block