x86/build: Move the install rule to arch/x86/Makefile

Currently, the install target in arch/x86/Makefile descends into
arch/x86/boot/Makefile to invoke the shell script, but there is no
good reason to do so.

arch/x86/Makefile can run the shell script directly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210729140023.442101-2-masahiroy@kernel.org