u-boot: add an option to indicate that DTC is needed

Some U-Boot configurations require the Device Tree compiler to be
available, so we need to depend on host-dtc (example configuration:
zynq_zed).

However, we don't want to build it unconditionally, since the vast
majority of U-Boot configurations don't need it (and host-dtc itself
has a bunch of dependencies).

So, we simply add a Config.in option that allows users to indicate
whether their U-Boot needs DTC or not, and depend on host-dtc if this
option is enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>