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/arm/Kconfig

@@ -15,6 +15,7 @@ config ARM_VIRT
     select ACPI
     select ARM_SMMUV3
     select GPIO_KEY
+    select DEVICE_TREE
     select FW_CFG_DMA
     select PCI_EXPRESS
     select PCI_EXPRESS_GENERIC_BRIDGE
@@ -265,6 +266,7 @@ config SBSA_REF
     default y
     depends on TCG && AARCH64
     imply PCI_DEVICES
+    select DEVICE_TREE
     select AHCI
     select ARM_SMMUV3
     select GPIO_KEY
@@ -347,6 +349,7 @@ config VEXPRESS
     bool
     default y
     depends on TCG && ARM
+    select DEVICE_TREE
     select A9MPCORE
     select A15MPCORE
     select ARM_MPTIMER
@@ -492,6 +495,7 @@ config XLNX_ZYNQMP_ARM
     select CPU_CLUSTER
     select DDC
     select DPCD
+    select DEVICE_TREE
     select SDHCI
     select SSI
     select SSI_M25P80
@@ -509,6 +513,7 @@ config XLNX_VERSAL
     depends on TCG && AARCH64
     select ARM_GIC
     select CPU_CLUSTER
+    select DEVICE_TREE
     select PL011
     select CADENCE
     select VIRTIO_MMIO