Add XZ Support

decompressors: add XZ decompressor module

    In userspace, the .lzma format has become mostly a legacy file format that
    got superseded by the .xz format.  Similarly, LZMA Utils was superseded by
    XZ Utils.

    These patches add support for XZ decompression into the kernel.  Most of
    the code is as is from XZ Embedded <http://tukaani.org/xz/embedded.html>.
    It was written for the Linux kernel but is usable in other projects too.

    Advantages of XZ over the current LZMA code in the kernel:
      - Nice API that can be used by other kernel modules; it's
        not limited to kernel, initramfs, and initrd decompression.
      - Integrity check support (CRC32)
      - BCJ filters improve compression of executable code on
        certain architectures. These together with LZMA2 can
        produce a few percent smaller kernel or Squashfs images
        than plain LZMA without making the decompression slower.

    This patch: Add the main decompression code (xz_dec), testing module
    (xz_dec_test), wrapper script (xz_wrap.sh) for the xz command line tool,
    and documentation.  The xz_dec module is enough to have a usable XZ
    decompressor e.g.  for Squashfs.