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