squashfs: Add lz4 hc compression

lz4 offers a high compression mode. A minimalistic file system shrinks by about 15%.

[Peter: Unconditionally use hc mode for lz4 instead of introducing another option]
Signed-off-by: Matthias Weisser <m.weisser.m@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>