kbuild: buildtar: install riscv compressed images as vmlinuz

Use the KBUILD_IMAGE variable to determine the right kernel image to
install and install compressed images to /boot/vmlinuz-$version like the
'make install' target already does.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>