Add build support for LZ4 ramdisks

Ramdisks are currently compressed using gzip, which is generally
smaller, but slower to decompress than lz4. Provide an option for an
lz4 scheme to reduce the time taken to unpack initramfs during boot.

Bug: 156129966
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: Iac9538e6ee6ec51e6b487de2101f53bb5d9c54c8