fix(build_library): Finish conversion to new bucket layout.

I started to move board files under a boards/ directory similar to how
the SDK is under sdk/ but didn't do so everywhere. This should finish
the job so everything is consistent now.

Note: This prefix is only used in developer and buildbot uploads. When
final releases are copied to $channel.release.core-os.net it doesn't use
the prefix since a) I already published urls without the prefix and b)
no sdk files are ever posted to the public release locations.