xtensa: move vmlinux.bin[.gz] to boot subdirectory

vmlinux.bin and vmlinux.bin.gz are always rebuilt in the kernel build
process. Add them to 'targets' and move them to the boot subdirectory
where their rules are. Update make rules that refer to them.

Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>