Move and rename elements

Note: !41 (closed) was combined into this MR to avoid an additional rebuild of live-bootstrap.

See commit messages for the included changes.

Main changes:

• Rename sysa.bst to live-bootstrap-stage1.bst – sysa.bst's final artifact wasn't actually the sysa sysroot, it's sysc's initial sysc_image. The previous nam le wqs misleading.
• Rename sysc.bst to live-bootatrap.bst – We no longer collect sysc_image in sysc.bst as-is, so sysc.bst is actually a composition of packages created by live-bootstrap. It's best not to call it sysc at this stage.
• Move base-sdk outside bootstrap/build – base-sdk is not conceptually part of bootstrap/build, both bootstrap and bootstrap/build use it directly. (we reached a similar conclusion in freedesktop-sdk)

A few other reasons for the changes:

• Element file names were getting too long
• Having both freedesktop-sdk-binary-seed and freedesktop-sdk repos use similar directory structures.