This brings U-Boot's cpp invocation into line with the way the Linux
kernel invokes cpp on device trees. Consistency will be useful to ensure
*.dts is portable between the two.
-undef also has the added advantage of not defining "linux", so DT
property names such as "linux,keymap" don't get mangled.
Signed-off-by: Stephen Warren <firstname.lastname@example.org>
Acked-by: Simon Glass <email@example.com>