initramfs: read CONFIG_RD_ variables for initramfs compression

When expert configuration option(CONFIG_EXPERT) is enabled, menuconfig
offers a choice of compression algorithm to compress initial ramfs image;
This choice is stored into CONFIG_RD_* variables.  But usr/Makefile uses
earlier INITRAMFS_COMPRESSION_* macros to build initial ramfs file.  Since
none of them is defined, resulting 'initramfs_data.cpio' file remains
un-compressed.

This patch updates the Makefile to use CONFIG_RD_* variables and adds
support for LZ4 compression algorithm.  Also updates the
'gen_initramfs_list.sh' script to check whether a selected compression
command is accessible or not.  And fall-back to default gzip(1)
compression when it is not.

Signed-off-by: P J P <prasad@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
scripts/gen_initramfs_list.sh: fix flags for initramfs LZ4 compression

LZ4 as implemented in the kernel differs from the default method now
used by the reference implementation of LZ4.  Until the in-kernel method
is updated to support the new default, passing the legacy flag (-l) to
the compressor is necessary.  Without this flag the kernel-generated,
LZ4-compressed initramfs is junk.

Kyungsik said:

: It seems that lz4 supports legacy format with the same option as lz4c
: does.  Just looking at the first few bytes of lz4 compressed image, we can
: see whether it is new format or not.
:
: It shows new format magic number without this patch.  New format magic
: number is 0x184d2204.
:
: $ hexdump -C ./initramfs_data.cpio.lz4 |more
: 00000000  04 22 4d 18 64 70 b9 69 (Little Endian)
: ...
:
: Currently kernel supports legacy format only.

Signed-off-by: Daniel M. Weeks <dan@danweeks.net>
Cc: Michal Marek <mmarek@suse.cz>
Acked-by: Kyungsik Lee <kyungsik.lee@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>