hw/mips/boston: Support dumpdtb monitor commands

hw/mips/boston: Support dumpdtb monitor commands

The boston machine doesn't set MachineState::fdt to the DTB blob that
it has loaded or created, which means that the QMP/HMP dumpdtb
monitor commands don't work.

Setting MachineState::fdt is easy in the non-FIT codepath: we can
simply do so immediately before loading the DTB into guest memory.
The FIT codepath is a bit more awkward as currently the FIT loader
throws away the memory that the FDT was in after it loads it into
guest memory.  So we add a void *pfdt argument to load_fit() for it
to store the FDT pointer into.

There is some readjustment required of the pointer handling in
loader-fit.c, so that it applies 'const' only where it should (e.g.
the data pointer we get back from fdt_getprop() is const, because
it's into the middle of the input FDT data, but the pointer that
fit_load_image_alloc() should not be const, because it's freshly
allocated memory that the caller can change if it likes).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250206151214.2947842-5-peter.maydell@linaro.org