testimage: enable ovmf support

Add support for running wic images with EFI as testimage.
Introduces a variable called QEMU_USE_OVMF for configuration.

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3af8aaff)
Signed-off-by: Steve Sakoman <steve@sakoman.com>