mx1fs2/flash: Fix multiple compiler warnings

"pointer targets in assignment differ in signedness"

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

board/mx1fs2/flash.c

@@ -173,7 +173,7 @@ flash_print_info(flash_info_t * info)
 	int i;
 	uchar *boottype;
 	uchar *bootletter;
-	uchar *fmt;
+	char *fmt;
 	uchar botbootletter[] = "B";
 	uchar topbootletter[] = "T";
 	uchar botboottype[] = "bottom boot sector";