fix build with make 3.80

According to Documentation/Changes, make 3.80 is still being supported
for building the kernel, hence make files must not make (unconditional)
use of features introduced only in newer versions.

Commit 1bf49dd4be0b ("./Makefile: export initial ramdisk compression
config option") however introduced "else ifeq" constructs which make
3.80 doesn't understand.  Replace the logic there with more conventional
(in the kernel build infrastructure) list constructs (except that the
list here is intentionally limited to exactly one element).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: 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

@@ -732,19 +732,13 @@ 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 ($(CONFIG_RD_LZ4), y)
-        INITRD_COMPRESS=lz4
-endif
-export INITRD_COMPRESS
+INITRD_COMPRESS-y                  := gzip
+INITRD_COMPRESS-$(CONFIG_RD_BZIP2) := bzip2
+INITRD_COMPRESS-$(CONFIG_RD_LZMA)  := lzma
+INITRD_COMPRESS-$(CONFIG_RD_XZ)    := xz
+INITRD_COMPRESS-$(CONFIG_RD_LZO)   := lzo
+INITRD_COMPRESS-$(CONFIG_RD_LZ4)   := lz4
+export INITRD_COMPRESS := $(INITRD_COMPRESS-y)
 
 ifdef CONFIG_MODULE_SIG_ALL
 MODSECKEY = ./signing_key.priv