kconfig: express dependency of individual boards on libfdt

kconfig: express dependency of individual boards on libfdt

Now that boards are enabled by default and the "CONFIG_FOO=y"
entries are gone from configs/devices/, there cannot be any more
a conflicts between the default contents of configs/devices/
and a failed "depends on" clause.

With this change, each individual board or target can express
whether it needs FDT.  It can then include the common code in the
build via "select DEVICE_TREE", which will also as tell meson to link
with libfdt.

This allows building non-microvm x86 emulators without having
libfdt available.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>