linux: handle read-only dts files

Some fine version control systems make all files read-only. The custom DTS file
may therefore be read-only, and that permission is preserved when copying into
the Linux build directory. A subsequent rebuild tries to 'cp' again, which
fails with a "Permission denied" error unless the -f option is used.

Signed-off-by: Hollis Blanchard <hollis_blanchard@mentor.com>
Acked-by: Nikolay Dimitrov <picmaster@mail.bg>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>