[flight_plan] add binding to ABI messages from flight plan (#2106)

This feature allows to create an automatic binding to a ABI message
while specifying the list of variable names ('_' are ignored).
A local (static) variable is updated to the latest value each time a new
message is received and it can be read in any flight plan instruction.

Example of usage:

 <variables>
  <abi_binding name="VELOCITY_ESTIMATE" vars="_, vx, vy, vz, _"/>
 </variables>

please not the in this example, the first and the last variable are
ignored but the number of variable must match the number of fields in
ABI messages description (conf/abi.xml).

An extra attribute 'id' allows to set a specific sender ID (by default
ABI_BROADCAST).

conf/flight_plans/flight_plan.dtd

@@ -14,8 +14,9 @@
 <!ELEMENT corner EMPTY>
 <!ELEMENT kml EMPTY>
 
-<!ELEMENT variables (variable*)>
+<!ELEMENT variables (variable|abi_binding)*>
 <!ELEMENT variable EMPTY>
+<!ELEMENT abi_binding EMPTY>
 
 <!ELEMENT modules (module*)>
 <!ELEMENT module  (configure|define)*>
@@ -114,6 +115,11 @@ alt_unit CDATA #IMPLIED
 alt_unit_coef CDATA #IMPLIED
 values CDATA #IMPLIED>
 
+<!ATTLIST abi_binding
+name CDATA #REQUIRED
+vars CDATA #REQUIRED
+id CDATA #IMPLIED>
+
 <!ATTLIST modules>
 
 <!ATTLIST module