Makefile: remove generated boards.cfg within make distclean

Signed-off-by: Roger Meier <roger@bufferoverflow.ch>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/MAKEALL b/MAKEALL
index 929fe88..392ea8d 100755
--- a/MAKEALL
+++ b/MAKEALL
@@ -658,7 +658,7 @@
 		MAKE="${MAKE} O=${output_dir}"
 	fi
 
-	${MAKE} distclean >/dev/null
+	${MAKE} mrproper >/dev/null
 
 	echo "Building ${target} board..."
 	${MAKE} -s ${target}_defconfig >/dev/null