dtc: Make make print a message when linking testcases

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> Currently, dtc relies on make's implicit rule to build the testcases.
> This means that when not making verbosely (V=0, the default) there is
> no message at all while relinking the testsuites.  This can be very
> confusing when updating libfdt.a (upon which the testcases depend) and
> make appears to do nothing.
> 
> This patch corrects the situation, borrowing the rule used to link dtc
> itself to link all the testcases as well.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl