Add support for zip archives, extraction paths

[darcagn]

As it stands, we can only extract tarballs when downloading and unpacking archives -- no ZIP support.

• Added support for checking archive filename for ZIP and extracting using unzip if so; else we use tar xf like we currently do.
• Unlike tarballs which usually contain a top level dir, ZIPs usually extract within the current working directory, so I added EXTRACT_DIR option for port Makefiles to specific the dir to extract to.
• Removed UNPACK_CMD option from config.mk. I doubt it was ever used, we depend upon tar anyway, we specify it as a dependency, and since scripts are using arguments that are passed to tar, it doesn't make much sense to configure this, since arguments are unlikely to work with any other tool.

[gyrovorbis]

I approve of this and it sounds great, but... I am complete ass at this bash scripty stuff, so I am not qualified to approve of the code review... just cheerleading.... XD

[QuzarDC]

You mentioned in the discords that this is meant to be for supporting FatFs. Could you include that in with this then? Otherwise it's a bit 'well, why should we bother when nothing is using it' and it's very difficult to validate without trying to modify one of the existing ports or creating another new port that would use it.