core-image-base: drop 'splash' from IMAGE_FEATURES

We use core-image-base as a basic non-graphical image, so we don't want
a splashscreen (not all the BSPs even have a framebuffer at this point).

JIRA: SB-3245

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
local.conf.sample: mention the 'splash' image feature

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #265 from kergoth/sb-3245

Disable inclusion of psplash in core-image-base