Makefile : fix cleaning rules

in order to leave the src tree as clean as before a build
+ add rm -f of autogenerated sim_core_config.h & sim_core_decl.h
  in simavr/Makefile
+ add rm *.hex in example/board_*/Makefile

examples/board_ledramp/Makefile

@@ -46,4 +46,4 @@ ${target}: ${board}
 	@echo $@ done
 
 clean: clean-${OBJ}
-	rm -rf *.a *.axf ${target} *.vcd
+	rm -rf *.a *.axf ${target} *.vcd *.hex

examples/board_simduino/Makefile

@@ -47,4 +47,4 @@ ${target}: ${board}
 	@echo $@ done
 
 clean: clean-${OBJ}
-	rm -rf *.a *.axf ${target} *.vcd
+	rm -rf *.a *.axf ${target} *.vcd *.hex

examples/board_timer_64led/Makefile

@@ -47,4 +47,4 @@ ${target}: ${board}
 	@echo $@ done
 
 clean: clean-${OBJ}
-	rm -rf *.a *.axf ${target} *.vcd
+	rm -rf *.a *.axf ${target} *.vcd *.hex

simavr/Makefile

@@ -83,6 +83,7 @@ ${target}	: ${OBJ}/${target}.elf
 
 clean: clean-${OBJ}
 	rm -rf ${target} *.a *.so
+	rm -f sim_core_*.h
 
 DESTDIR = /usr/local
 PREFIX = ${DESTDIR}