adt-installer: Allow changing YOCTOADT_REPO

adt-installer: Allow changing YOCTOADT_REPO

YOCTOADT_REPO within adt-installer.conf was hardcoded to
http://adtrepo.yoctoproject.org/${SDK_VERSION}

This allows it to be overriden in local.conf by setting
ADTREPO = "http://adtrepo.yoctoproject.org/${SDK_VERSION}"

I'm also fixing a bad rm in do_populate_adt. The file name
that is being rm'd is wrong.

Signed-off-by: Elizabeth Flanagan <elizabeth.flanagan@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>