./Makefile: export initial ramdisk compression config option

Make menuconfig allows one to choose compression format of an initial
ramdisk image.  But this choice does not result in duly compressed ramdisk
image.  Because - $ make install - does not pass on the selected
compression choice to the dracut(8) tool, which creates the initramfs
file.  dracut(8) generates the image with the default compression, ie.
gzip(1).

This patch exports the selected compression option to a sub-shell
environment, so that it could be used by dracut(8) tool to generate
appropriately compressed initramfs images.

There isn't a straightforward way to pass on options to dracut(8) via
positional parameters.  Because it is indirectly invoked at the end of a $
make install sequence.

 # make install
   -> arch/$arch/boot/Makefile
    -> arch/$arch/boot/install.sh
     -> /sbing/installkernel ...
      -> /sbin/new-kernel-pkg ...
       -> /sbin/dracut ...
Signed-off-by: P J P <ppandit@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Makefile

@@ -720,6 +720,22 @@ mod_strip_cmd = true
 endif # INSTALL_MOD_STRIP
 export mod_strip_cmd
 
+# Select initial ramdisk compression format, default is gzip(1).
+# This shall be used by the dracut(8) tool while creating an initramfs image.
+#
+INITRD_COMPRESS=gzip
+ifeq ($(CONFIG_RD_BZIP2), y)
+        INITRD_COMPRESS=bzip2
+else ifeq ($(CONFIG_RD_LZMA), y)
+        INITRD_COMPRESS=lzma
+else ifeq ($(CONFIG_RD_XZ), y)
+        INITRD_COMPRESS=xz
+else ifeq ($(CONFIG_RD_LZO), y)
+        INITRD_COMPRESS=lzo
+else ifeq ($(CONFIG_RD_LZ4), y)
+        INITRD_COMPRESS=lz4
+endif
+export INITRD_COMPRESS
 
 ifdef CONFIG_MODULE_SIG_ALL
 MODSECKEY = ./signing_key.priv

init/do_mounts_rd.c

@@ -57,6 +57,11 @@ static int __init crd_load(int in_fd, int out_fd, decompress_fn deco);
  *	cramfs
  *	squashfs
  *	gzip
+ *	bzip2
+ *	lzma
+ *	xz
+ *	lzo
+ *	lz4
  */
 static int __init
 identify_ramdisk_image(int fd, int start_block, decompress_fn *decompressor)