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>