linux: install dtc (device tree compiler) as host tool if selected

Having dtc as a host tool can be useful for users that have a custom
boot scenario where the device tree is not embedded in the kernel.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>