PPC: e500: Move to u-boot as firmware

Almost all platforms QEMU emulates have some sort of firmware they can load
to expose a guest environment that closely resembles the way it would look
like on real hardware.

This patch introduces such a firmware on our e500 platforms. U-boot is the
default firmware for most of these systems and as such our preferred choice.

For backwards compatibility reasons (and speed and simplicity) we skip u-boot
when you use -kernel and don't pass in -bios. For all other combinations like
-kernel and -bios or no -kernel you get u-boot as firmware.

This allows you to modify the boot environment, execute a networked boot through
the e1000 emulation and execute u-boot payloads.

Signed-off-by: Alexander Graf <agraf@suse.de>
