psplash-portrait-config: add recipe

Add psplash-portrait-config recipe to install a systemd unit override
for psplash-start.service that forces a 90 degree rotation for
portrait mode, add add it to packagegroup-agl-demo-platform so that
it takes affect in the agl-demo-platform image.

Bug-AGL: SPEC-4433

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ida64df27a3a3c22bfe2eae8233ff8656800e23d0