linux: provide symlink dtc->linux-dtc is there is no dtc yet

linux: provide symlink dtc->linux-dtc is there is no dtc yet

Commit ab74e09eb4e28dab8bed8d783c5f0464d39a32e7 renamed the dtc host tool
provided by linux to linux-dtc to avoid clashes with the dtc host tool
provided by host-dtc.

However, external scripting may well rely on the existence of a device tree
compiler as $(HOST_DIR)/usr/bin/dtc, regardless of its source. Changing
these external scripts to use linux-dtc means that the scripts need to be
aware of the buildroot release they are working with, which is not very
nice.

Add a symlink dtc->linux-dtc when no $(HOST_DIR)/usr/bin/dtc is present.
When host-dtc is not enabled, the end result will be dtc and
linux-dtc representing the same thing.
When host-dtc is enabled, either it is build before linux and no symlink
is created at any time, or it is build after linux, and the 'install'
command in host-dtc will overwrite the symlink with a proper dtc. In both
cases, the end result will be dtc and linux-dtc representing a different
thing.

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