refactor(rpi): move more platform-specific code into common

In preparation for RPi 5 support, which will reuse most of the RPi 4
logic except for DTB patching.

Change-Id: I6f6ef96933711a1798757a3389adae1b8ee3de6c
Signed-off-by: Mario Bălănică <mariobalanica02@gmail.com>
7 files changed