Copter: RTL at no less than 2m above home alt

ArduCopter/config.h

@@ -467,6 +467,10 @@
  # define RTL_ALT 				    1500    // default alt to return to home in cm, 0 = Maintain current altitude
 #endif
 
+#ifndef RTL_ALT_MIN
+ # define RTL_ALT_MIN               200     // min height above ground for RTL (i.e 2m)
+#endif
+
 #ifndef RTL_LOITER_TIME
  # define RTL_LOITER_TIME           5000    // Time (in milliseconds) to loiter above home before begining final descent
 #endif

ArduCopter/control_rtl.pde

@@ -389,6 +389,7 @@ static float get_RTL_alt()
 {
     // maximum of current altitude and rtl altitude
     float rtl_alt = max(current_loc.alt, g.rtl_altitude);
+    rtl_alt = max(rtl_alt, RTL_ALT_MIN);
 
 #if AC_FENCE == ENABLED
     // ensure not above fence altitude if alt fence is enabled