Add support for BTRFS Fillesystem

[shiren00]

We would appreciate it if anyone with additional materials or actual disk image files related to this could share them :D

• https://en.wikipedia.org/wiki/Btrfs

• https://btrfs.readthedocs.io/en/latest/dev/dev-btrfs-design.html

• extractor: https://github.com/shujianyang/btrForensics

[devttys0]

WIP: https://github.com/ReFirmLabs/binwalk/tree/btrfs_support

[devttys0]

BTRFS identification is working and merged into the master branch. Unfortunately, there doesn't seem to be an easy existing solution to extracting the files from a BTRFS image (the btrForensics tool above is interactive, not automated, and requires knowing the inode number of each file you want to extract; it also fails on BTRFS images created with default mkfs.btrfs settings and doesn't look like it's been updated in several years).

Since writing a custom extractor would require a good deal more time, and I'm trying to push a stable release of Binwalk this week, I'm closing this issue and I've added issue #724 to specifically write a BTRFS extractor.