dbsfed.xsd: make kinitrd optional

it makes no sense on the <project><buildimg> position, but is needed if a
<pkg-list> for the buildimg will be defined.

Signed-off-by: Manuel Traut <manut@linutronix.de>
rfs: configure serial console - print warning if it can't be parsed

Signed-off-by: Manuel Traut <manut@linutronix.de>
buildchroot: remove FutureWarning printed by etree.py

/home/local/projects/elbe/elbe/elbepack/treeutils.py:69: FutureWarning: This search is broken in 1.3 and earlier, and will be fixed in a future version.  If you rely on the current behaviour, change it to './project/mirror/url-list'
  return not (self.et.find(path) is None)

Signed-off-by: Manuel Traut <manut@linutronix.de>