This adds a bunch of unit tests for the "fdt apply" command.

They've all been run successfully in the sandbox. However, as you still
require an out-of-tree dtc with overlay support, this is disabled by
default.

Acked-by: Simon Glass <[email protected]>
Acked-by: Pantelis Antoniou <[email protected]>
Signed-off-by: Maxime Ripard <[email protected]>