Add lzo, lz4 compression and read INITRD_COMPRESS

This patch adds support for lzop(1) & lz4(1) compression
algorithms to compress iniramfs image file. Both are supported
by the Linux kernel.

Linux kernel exports user's choice of initramfs compression
algorithm as a shell environment variable: INITRD_COMPRESS.
This patch adds support to read this variable and duly compress
the initramfs image file.

Environment variable INITRD_COMPRESS has less precedence than the
command line options --gzip, etc. Ie. command line options could
override the compression algorithm defined by $INITRD_COMPRESS.

Signed-off-by: P J P <ppandit@redhat.com>

[Edited-by: Harald Hoyer: add documentation about lzo and lz4]