fix build with make 3.80

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 b5d917c043de ("./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>