Log: Restore an interface target for logging header

The tfm_ns_log target has been removed but it also carried the
configuration flags for the NS logging. Restore it partially
as an INTERFACE target that is linked to tfm_api_ns which grants
for it to be correctly propagated to all targets that need to
log through the TF-M provided NS interface lib

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I4c188e30dc58f43d7f188c90056199546ed0df8f
1 file changed