Super easy, just need to install rEFInd (http://www.rodsbooks.com/refind/) . Some OS upgrades might hide partitions again but you can make it persist trough updates with a script. ALL hhe info is on the page.
You can also hold down the Option key when booting to select bootable partitions or external devices. Different models may require extra steps to enable this, but I've never needed to use rEFInd.