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>

hw/openrisc/Kconfig

@@ -2,6 +2,7 @@ config OR1K_SIM
     bool
     default y
     depends on OPENRISC
+    select DEVICE_TREE
     select SERIAL
     select OPENCORES_ETH
     select OMPIC
@@ -14,6 +15,7 @@ config OR1K_VIRT
     imply PCI_DEVICES
     imply VIRTIO_VGA
     imply TEST_DEVICES
+    select DEVICE_TREE
     select PCI
     select PCI_EXPRESS_GENERIC_BRIDGE
     select GOLDFISH_RTC