add PDK_PLATFORM_ZIP_PRODUCT_BINARIES to allow adding product specific binaries to platform.zip

- This can be used to deliver stuffs like kernel symbol file.
- Files are added in relative path from the top of android source tree.
- PDK_PLATFORM_ZIP_PRODUCT_BINARIES will be defined in BoardConfig.mk.
- files under out dir will be rejected to prevent possible conflicts.

bug: 9963432
Change-Id: Id1da643534cd86939df4941f39e4414d343bba99