psplash: Move logo in an assets directory

In this way, multiple recipes can take advantage of various assets (eg.
images, logos).

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

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

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