Resurrect framebuffer code from previous port

[r1mikey]

Resurrect the EFI framebuffer code, as expressed via xboot_info, from my original port. This would integrate into eboot (the moved PSM loader), massaging the firmware-provided framebuffer into something the kernel can use.

Update the inetboot loaders to use a common codepath that arranges for the simplefb FDT node to be used as the framebuffer, then sets up xboot_info to communicate that FB to unix.

Update unix to use the passed framebuffer. The framebuffer code currently uses early boot mappings for the shadow framebuffer, which will need a careful look, since this is now very different from my original port (it should still work, but not the same as it used to).

NOTE: Will also need proper "final" mappings of both the framebuffer and the shadow area, done once we've set up the VM subsystem in os/startup.c.