Make sure WIN_BUILD_OUTPUT is set when copying Windows build output

Without this check, the xctools step tries to rsync everything from the
root directory into the build tree.
Make sure REPO_PROD_CACERT and REPO_DEV_CACERT are set

Without these checks, resolve_path incorrectly resolves these empty
variables as references to the top-level directory in the build tree.
These pass the check in check_repo_signing_file because the directory
exists, then the xenclient-repo-certs recipe uses them in its SRC_URI
and tries to copy the whole build tree under its working directory.

Also move all the checks for these variables earlier in do_oe_setup,
before it tests whether xenclient/conf/local.conf exists. Otherwise, if
the setupoe step fails on one of these checks, rerunning it won't
generate a usable xenclient/conf/local.conf file.
Merge pull request #9 from rickleeming/master

Fix error checking for WIN_BUILD_OUTPUT and REPO_*_CACERT