Add native cpio extractor

[nmatt0]

Problem

Binwalk currently uses the external 7zz tool to extract CPIO archives. However, 7zz does not support ASCII CPIO archives (SVR4 format), causing extraction failures with the error "Cannot open the file as archive". This affects firmware images that use initramfs embedded in kernel images, which commonly use this CPIO format.

Solution

Implemented a native internal CPIO extractor following the existing pattern used by the RomFS extractor.

I can privately provide the firmware file that was the test case for this change.

[impervius]

Are you sure that this is a real problem? I can't reproduce it.

I am creating an archive in this way

$ find . | cpio -o -H newc > /tmp/test_file
$ file test_file
test_file: ASCII cpio archive (SVR4 with no CRC)

And binwalk extracts it with no issues at all.