Makefile: clean boot.bin

When build for Atmel related boards which support SPL,
it will generate boot.bin, also clean when it when do
"make clean" operation.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -1278,7 +1278,7 @@ CLEAN_DIRS  += $(MODVERDIR) \
 			$(filter-out include, $(shell ls -1 $d 2>/dev/null))))
 
 CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h \
-	       u-boot* MLO* SPL System.map
+	       boot* u-boot* MLO* SPL System.map
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config include/generated spl tpl \