Support selecting branches/tags for KallistiOS

[darcagn]

At the moment, DreamSDK only allows selecting a specific repository for KallistiOS, with the main project Github/SourceForge being selectable. However, it will always pull master. KallistiOS is now on roughly 6-month release cycles of stable builds, with master being designated unstable, so DreamSDK should be able to select a particular tag so that projects can use the stable release, which is currently recommended by KallistiOS.

Besides allowing greater choice, another reason for this is an issue could arise if KallistiOS master makes mandatory changes to its toolchain. Since DreamSDK does not offer a built-in mechanism to rebuild the toolchain, users would be stuck with an outdated toolchain that would be incompatible with master.