linux: Make dtc install step more reliable

Checking for the existence of the dtc binary built by the
non-dependent dtc package may cause instable behaviour when giving more
freedom on the order of how the packages are built (parallelization).

In addidion, when moving to per-package host/target method, the check
would always trigger in the isolated host, leading to linux-dtc always
being installed as dtc.
This in turn may lead to undesired overwriting of the real host-dtc binary
when finally assembling the global host dir.

Thus rework the linux-dtc install condition to be defined by configuration
rather than compile time order.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>