s390: move the install rule to arch/s390/Makefile

Currently, the install target in arch/s390/Makefile descends into
arch/s390/boot/Makefile to invoke the shell script, but there is no
good reason to do so.

arch/s390/Makefile can run the shell script directly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210729142338.446002-1-masahiroy@kernel.org


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>