Multiple screen support in eglfs for embedded devices

As of today eglfs is incapable of supporting multiple screens. It will always use fb0, which is not ideal since boards like the iMX6 are often used in a multi-display setup.

Both eglfs and the iMX6 hooks need to be enhanced. At minimum the framebuffer device/index has to be configurable, so that, similarly to linuxfb, the display to be used can be specified on the command-line. Once that's done, we need to consider discovering and exposing all of the available FBs as QPlatformScreens. This might introduce some unforeseen difficulties, though, and needs some investigation.