unpackbootimg: Add support for detecting lz4 compressed ramdisks

This will check for lz4 magic, and if found change the extension of the
file. Else it'll fall back to the default behavior of assuming gzip. This has
been tested with stock LS980 boot images and LS970 boot images for lz4
and gzip respectively.

Change-Id: If2139ff172397b6db079ffb7ab9cb61897c38fb3