image: Include psplash only when screen machine feature is enabled

base-image boots in degraded mode when using systems without display
system since there is no fb device detected and pslash service would
fail to start. Removing this image feature means that core-image-base is
complete for headless devices

[RP: tweak the false case so it builds]
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>