configure: Report unknown target names more helpfully

If the user specifies a target list themselves, check each entry
to make sure it's a target we recognise. This allows us to print
a helpful error message, rather than falling through (where we
would probably eventually end up hitting the uninformative
"ERROR: Unsupported target CPU").

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1369062976-301-3-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
