Detect conflicting profiling attributes

Detect situations where content is accidentally excluded from the build
because the os profiling of one element contradicts that of its
children. For example:

<itemisedlist os="opensuse;sles">
  <listitem os="ubuntu;debian">

The listitem in this example can never be displayed as the os conditions
it specifies are not a subset of those specified by its parent.

Change-Id: I17fb2e249210052d3c166d85533f72fa491f955c
Merge "Detect conflicting profiling attributes"