Makefile: make zbm-release the default action

[classabbyamp]

this is similar to many other projects that are install-only, like xtools

[classabbyamp]

alternatively, we could move zbm-release up so it's the default target, as that's what the void template does: https://github.com/void-linux/void-packages/blob/master/srcpkgs/zfsbootmenu/template#L6

[ahesford]

I like moving the zbm-release target up to the default slot. If you want to keep a no-op target, I'd prefer a name like dummy or no-op over all, which implies it will run other functional targets.