psplash: add override for our own psplash branding

Using the OSTC logo until we have the real OpenHarmony logo in place
later.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

meta-ohos-core/recipes-core/psplash/psplash_%.bbappend

+FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
+SPLASH_IMAGES = "file://ostc-logo-small.png;outsuffix=default"