um: Unconditionally call unflatten_device_tree()

Call this function unconditionally so that we can populate an empty DTB
on platforms that don't boot with a command line provided DTB. There's
no harm in calling unflatten_device_tree() unconditionally. If there
isn't a valid initial_boot_params dtb then unflatten_device_tree()
returns early.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: linux-um@lists.infradead.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240217010557.2381548-4-sboyd@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

um: Fix order of dtb unflatten/early init

Scan the tree for reserved memory before unflattening it. As Rob
Herring said:
| These should be reversed. early_init_fdt_scan_reserved_mem() works on
| the flat tree. Reserved memory needs to be reserved before
| unflatten_device_tree() starts allocating memory. Though I imagine that
| doesn't really matter for UML.

Suggested-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/lkml/YeF%2FVbxo6fAt0WLp@robh.at.kernel.org/
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

um: Add devicetree support

Add a dtb=<filename> option to boot UML with a devicetree blob. This
can be used for testing driver code using UML.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
[rw: Add dependency on CONFIG_OF]
Signed-off-by: Richard Weinberger <richard@nod.at>