dhcpcd: correct ccache-fixing patch for Xtensa

The Xtensa gcc does not seem to like output on /dev/null. The applied patch
on dhcpcd is thus unusable, and a more verbose version involving a temporary
file is needed.

Reported-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>