initramfs: make compression options not depend on INITRAMFS_SOURCE

Even if INITRAMFS_SOURCE is empty, usr/gen_initramfs.sh generates a
tiny default initramfs, which is embedded in vmlinux.

So, defining INITRAMFS_COMPRESSION* options should be valid irrespective
of INITRAMFS_SOURCE.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>