configure: dtc: Probe for libfdt_env.h

configure: dtc: Probe for libfdt_env.h

Currently QEMU provides a local clone of the file libfdt_env.h in
/include. This file is supposed to come with the libfdt package and is
only needed for broken installs of libfdt. Now that we have submodule
dtc, just ignore these broken installs and prompt for the dtc submodule
install instead. QEMU's local libfdt_env.h is removed accordingly.

Manifests as a bug when building QEMU with modern libfdt. The new
version of libfdt does not compile when QEMUs libfdt_env.h takes
precedence over the hosts.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 9b6a3a52e3f46cfbc1ded9ab56385ec045e46705.1369628289.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>