kconfig: allow compiling out QEMU device tree code per target

Introduce a new Kconfig symbol, CONFIG_DEVICE_TREE, that specifies whether
to include the common device tree code in system/device_tree.c and to
link to libfdt.  For now, include it unconditionally if libfdt is
available.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/Kconfig.host b/Kconfig.host
index f6a2a13..a0d4a52 100644
--- a/Kconfig.host
+++ b/Kconfig.host
@@ -23,6 +23,10 @@
 config TPM
     bool
 
+config FDT
+    bool
+    select DEVICE_TREE
+
 config VHOST_USER
     bool