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

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

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

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