riscv: move the (z)install rules to arch/riscv/Makefile

Currently, the (z)install targets in arch/riscv/Makefile descend into
arch/riscv/boot/Makefile to invoke the shell script, but there is no
good reason to do so.

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

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>