Add trace library

Add a library which supports tracing of execution using built-in gcc
features and a microsecond timer. This can be used to record a list of
function which are executed, along with a timestamp for each. Later
this information can be sent to the host for processing.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/Makefile

@@ -71,6 +71,7 @@ COBJS-y += linux_string.o
 COBJS-$(CONFIG_REGEX) += slre.o
 COBJS-y += string.o
 COBJS-y += time.o
+COBJS-$(CONFIG_TRACE) += trace.o
 COBJS-$(CONFIG_BOOTP_PXE) += uuid.o
 COBJS-y += vsprintf.o
 COBJS-$(CONFIG_RANDOM_MACADDR) += rand.o