[RPI][SPLASH] enable splash screen on RaspberryPi

This merge request:

• patches U-boot to display the splash bmp from SD (splashsource)
• configures logo display with kernel cmdline args (fbcon)
• excludes bbappends meta-raspberrypi/recipes-core/psplash/ to display edgehog logo instead of raspberry
• configures some drivers as built-in kernel modules, otherwise psplash crashes and causes the OS to hang after boot