Makefile: Don't build shared libfdt

The submodule DTC should just build and use DTC as static (the whole
motivation for using submod is lack of widespread distro support).

The .so causes build failures on some platforms, so adjust the make
target to just build the static lib.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Makefile: Use QEMU_FLAGS for DTC compilation

Build DTC as QEMU is built. Fixes the issue with mingw build which can't
handles DTC's default CFLAGS.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>