Makefile: Add downloads/stage3-amd64-current.tar.bz2 rule
Makefile: Add downloads/stage3-amd64-current.tar.bz2 rule

Based on [1] and relicenced under the Apache License, Version 2.0.

[1]: https://github.com/wking/oci-gentoo-minimal/blob/5f6850f05a4c183434cdb77774b10c9719649d49/Makefile#L18-L20

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

wking/ocitools-v2,opencontainers/runtime-tools,liangchenye/ocitools,opencontainers/ocitools,wking/ocitools-v2,hmeng-19/ocitools,hmeng-19/ocitools,liangchenye/ocitools,opencontainers/runtime-tools,opencontainers/ocitools