arm/boot: split load_dtb() from arm_load_kernel()

arm/boot: split load_dtb() from arm_load_kernel()

load_dtb() depends on arm_load_kernel() to figure out place
in RAM where it should be loaded, but it's not required for
arm_load_kernel() to work. Sometimes it's neccesary for
devices added with -device/device_add to be enumerated in
DTB as well, which's lead to [1] and surrounding commits to
add 2 more machine_done notifiers with non obvious ordering
to make dynamic sysbus devices initialization happen in
the right order.

However instead of moving whole arm_load_kernel() in to
machine_done, it's sufficient to move only load_dtb() into
virt_machine_done() notifier and remove ArmLoadKernelNotifier/
/PlatformBusFDTNotifierParams notifiers, which saves us ~90LOC
and simplifies code flow quite a bit.
Later would allow to consolidate DTB generation within one
function for 'mach-virt' board and make it reentrant so it
could generate updated DTB in device hotplug secenarios.

While at it rename load_dtb() to arm_load_dtb() since it's
public now.

Add additional field skip_dtb_autoload to struct arm_boot_info
to allow manual DTB load later in mach-virt and to avoid touching
all other boards to explicitly call arm_load_dtb().

 1) (ac9d32e hw/arm/boot: arm_load_kernel implemented as a machine init done notifier)

Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Message-id: 1525691524[email protected]
Signed-off-by: Peter Maydell <[email protected]>