microblaze: Makefile cleanups

If CONFIG_INITRAMFS_SOURCE is set, "scripts/gen_initramfs_list.sh"
checks if the cpio image exists. Remove the duplicate check from the
Makefile.

Remove the "clean-kernel" variable which is unused in the Makefile and
is not used by the Kbuild.

Signed-off-by: Arun Bhanu <arun@bhanu.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>