Add JFFS2 Packer/Unpacker

[marczalik]

• I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)
This draft PR adds a JFFS2 packer/unpacker using the Jefferson Python package and mkfs.jffs2 from mtd-utils.

Link to Related Issue(s)

Please describe the changes in your request.
Add support to pack and unpack JFFS2 filesystems.

Note: I haven't been able to find a readily available mtd-utils package for Apple Silicon, so for M1 support it may need to be cross-compiled. Directions here (I haven't tested this yet).

Anyone you think should look at this, specifically?
@rbs-jacob

[EdwardLarson]

Changes look good! Can you merge master into this branch though first please? Since we just did a release and shifted the changelog around, the automatic merge (though there are no conflicts) is prone to incorrectly placing new changelog entries into sections for existing releases.

[marczalik]

Changes look good! Can you merge master into this branch though first please? Since we just did a release and shifted the changelog around, the automatic merge (though there are no conflicts) is prone to incorrectly placing new changelog entries into sections for existing releases.

Sure can!

[rbs-jacob]

One small change requested. One additional, general question that shouldn't block merging this.

[rbs-jacob]

Thanks for following up on all of the threads – looks good to me!