nios2: move the install rule to arch/nios2/Makefile

Currently, the install target in arch/nios2/Makefile descends into
arch/nios2/boot/Makefile to invoke the shell script, but it is no
good reason to do so.

arch/nios2/Makefile can run the shell script directly.

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