Instead of overriding the default psplash image, the raspberrypi image is added
as an additional image. It is then set as the default in the raspberrypi machine
config file so that other targets do not pick up our splash image.
Tested on raspberrpi and qemuarm: on raspberrypi I see to Raspberry Pi logo, on
qemuarm I see the Yocto Project logo.
Signed-off-by: Paul Barker <paul@paulbarker.me.uk>