armv7m: Guard against no -kernel argument

A -kernel argument must be specified for this machine. Guard against no -kernel
argument. Previously gave an unhelpful "bad address" error message.

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
