Fix指定BUILD_DIR时编译错误

修复指定obj输出目录时(如make O=./build )，cat合并bin不成功.
若不指定BUILD_DIR则和原来一致，结果输出到TOPDIR。否则结果保存到BUILD_DIR下
Merge pull request #1 from kangear/master

sync
Merge pull request #2 from wb0330/master

Fix指定BUILD_DIR时编译错误