unmkinitramfs: Add support for lzma compression
The kernel and mkinitramfs support lzma compression, but for some reason this was not included in unmkinitramfs.
- Add a format code, magic entry, and decompression command for lzma to unmkinitramfs
- Add lzma to the compression commands in the test case
Signed-off-by: Ben Hutchings benh@debian.org