To avoid keeping stale files in the scripts/dtc directory, care had to
be taken to delete all files except for the ones native to barebox.
Make this less error-prone by having careupdate-dtc-source.sh delete
these files as part of the update process.

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