Make bat_checker configurable via airframe file

conf/modules/bat_checker.xml

@@ -1,22 +1,21 @@
 <!DOCTYPE module SYSTEM "module.dtd">
 
+<!--
+     Battery checker module
+     @define BAT_CHECKER_DELAY Number of seconds the battery voltage has to be
+                               below LOW_BAT_LEVEL before the warning signal is
+                               activated.
+     @define BAT_CHECKER_LED   The LED to use for the warning signal.
+     -->
 <module name="bat_checker">
   <header>
     <file name="bat_checker.h"/>
   </header>
   <init fun="init_bat_checker()"/>
   <periodic fun="bat_checker_periodic()" freq="2" autorun="TRUE"/>
   <makefile>
-    <!-- the warning signal will be enabled when supply voltage is below
-         LOW_BAT_LEVEL for this number of seconds -->
-    <define name="BAT_CHECKER_DELAY" value="5"/>
-    <define name="BAT_CHECKER_LED" value="4"/>
     <define name="bat_checker_periodic_FREQ" value="2"/>
     <file name="bat_checker.c"/>
   </makefile>
-  <!--makefile target="demo">
-    <define name="SOME_FLAG"/>
-    <configure name="SOME_DEFINE" value="bla"/>
-  </makefile-->
 </module>
 

sw/airborne/modules/bat_checker/bat_checker.c

@@ -31,6 +31,19 @@
 #error You must define CRITIC_BAT_LEVEL to use this module!
 #endif
 
+#ifndef LOW_BAT_LEVEL
+#error You must define LOW_BAT_LEVEL to use this module!
+#endif
+
+#ifndef BAT_CHECKER_LED
+#error You must define BAT_CHECKER_LED in your airframe file.
+#endif
+
+#ifndef BAT_CHECKER_DELAY
+#warning BAT_CHECKER_DELAY is undefined. Falling back to 5 seconds.
+#define BAT_CHECKER_DELAY 5
+#endif
+
 // at this level, the buzzer will be activated periodically
 #define WARN_BAT_LEVEL1 (LOW_BAT_LEVEL*10)