hw: Centralize handling of -machine dumpdtb option

hw: Centralize handling of -machine dumpdtb option

Currently we handle the 'dumpdtb' machine sub-option ad-hoc in every
board model that has an FDT.  It's up to the board code to make sure
it calls qemu_fdt_dumpdtb() in the right place.

This means we're inconsistent and often just ignore the user's
command line argument:
 * if the board doesn't have an FDT at all
 * if the board supports FDT, but there happens not to be one
   present (usually because of a missing -fdt option)

This isn't very helpful because it gives the user no clue why their
option was ignored.

However, in order to support the QMP/HMP dumpdtb commands we require
now that every FDT machine stores a pointer to the FDT in
MachineState::fdt.  This means we can handle -machine dumpdtb
centrally by calling the qmp_dumpdtb() function, unifying its
handling with the QMP/HMP commands.  All the board code calls to
qemu_fdt_dumpdtb() can then be removed.

For this commit we retain the existing behaviour that if there
is no FDT we silently ignore the -machine dumpdtb option.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>