dtc: add host build

Having a host variant of dtc is needed for example for U-Boot, which
uses the Device Tree for a number of platforms.

In addition, now that we have a proper host-dtc package, it is no
longer needed for the linux package to install the host dtc compiler:
users interested in having the host dtc compiler can simply enable
this package.

A Config.in.host option is added to build host-dtc, because the
initial reason why the host DTC built by the kernel was installed in
$(HOST_DIR)/usr/bin (commit 707d44d0
from Thomas DS) was:

    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.

[Thomas:
  - rework the commit log.]
[Peter: keep the (renamed) linux version]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>