Unlike Linux:
- barebox' scripts/gen-dtb-s has a dependency on fdget,
  thus change the script to copy fdget.c as part of the update.
- has the libfdt source files in the same directory as the rest
- we don't have/need Makefile.libfdt

Adjust the update-dtc-source.sh script to account for this, so
it can be used to update the scripts/dtc directory. To do so, run
the script while its directory is otherwise empty.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>