Makefile: dtc: update, build the libfdt target

Makefile: dtc: update, build the libfdt target

dtc submodule update, now call the libfdt target from the new
dtc Makefile, which has been changed to not require bison, flex, etc.
This removes warnings during the build.

scripts/ symlink and tests directory creation are not necessary,
and neither is calling the clean rule explicitly.

Signed-off-by: Claudio Fontana <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <20200518160319[email protected]>
Message-Id: <20200612190237[email protected]>