SDK: support generating repository XML with latest XSD. DO NOT MERGE.

The latest version of the repository XSD changes:
- archive attributes (os/arch) are now element (host-os)
- system images are not longer in the repository.xml and
  get their own XML file.

(cherry picked from commit ea40d2f4c5af9744f7140a31d551a55dade128d9)

Change-Id: Id0363c446014a96a7f78c8e5500fae6092e97dc3
7 files changed