initramfs: display custom splash screen from boot partition
In some cases, it would be useful to display another image instead of the "postmarketOS loading" screen. Let's make it possible to override that image, if a splash screen with a certain file name is stored on the boot partition. For the error screens, I'd just leave the pmOS branding as it would become too complicated otherwise.
Useful for:
- postmarketos-ondev#28 (closed): let the pmOS installer install other distros with FDE
- possibly @MartijnBraam's factorytest