Keep binary compatilities TEECore <--> TA

In case the core is compiled with
    CFG_TEE_TA_LOG_LEVEL=0
then TA cannot be compiled with CFG_TEE_TA_LOG_LEVEL!=0 because of the trace
function.

This patch implements stub trace functions in case of unsufficient trace level.
It also check
- TRACE_LEVEL < 0  instead of TRACE_LEVEL == 0
- TRACE_LEVEL >= 0 instead of TRACE_LEVEL != 0
to take into account negative trae levels

Change-Id: I7b4d2d576c50e103d9cf6f5b22f9f99a1ab96d6a

Conflicts:
	core/arch/st231/kernel/tee_ta_manager.c
	lib/libutils/ext/include/trace.h