buildroot

fs/common.mk: support lz4 compression

Similar to the other compressors.  Notice that we use the -l (legacy format)
for Linux kernel initrd compatibility.

Lz4 decompression is supported by the Linux kernel since 3.11.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>