configure: Require libfdt for arm, ppc, microblaze softmmu targets

A number of our softmmu targets (PPC, ARM, Microblaze) now more or
less require flattened device tree support for various board models
to work correctly.  Make libfdt mandatory if the target list includes
these, rather than building unhelpful half-functional binaries.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Message-id: 1369409217-7553-2-git-send-email-peter.maydell@linaro.org

configure

@@ -2486,6 +2486,26 @@ fi
 
 ##########################################
 # fdt probe
+# fdt support is mandatory for at least some target architectures,
+# so insist on it if we're building those system emulators.
+fdt_required=no
+for target in $target_list; do
+  case $target in
+    arm*-softmmu|ppc*-softmmu|microblaze*-softmmu)
+      fdt_required=yes
+    ;;
+  esac
+done
+
+if test "$fdt_required" = "yes"; then
+  if test "$fdt" = "no"; then
+    error_exit "fdt disabled but some requested targets require it." \
+      "You can turn off fdt only if you also disable all the system emulation" \
+      "targets which need it (by specifying a cut down --target-list)."
+  fi
+  fdt=yes
+fi
+
 if test "$fdt" != "no" ; then
   fdt_libs="-lfdt"
   # explicitly check for libfdt_env.h as it is missing in some stable installs