Reactivate the tracing feature

This was lost sometime in the Kbuild conversion. Add it back.

Check that the trace test now passes:

$ ./test/trace/test-trace.sh
Simple trace test / sanity check using sandbox

/tmp/filemHKPGw
Build sandbox
O=sandbox FTRACE=1
  GEN     /home/sjg/c/src/third_party/u-boot/files/sandbox/Makefile
Configuring for sandbox board...
Check results
Test passed

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

config.mk

@@ -53,6 +53,10 @@ ifdef	BOARD
 sinclude $(srctree)/board/$(BOARDDIR)/config.mk	# include board specific rules
 endif
 
+ifdef FTRACE
+PLATFORM_CPPFLAGS += -finstrument-functions -DFTRACE
+endif
+
 #########################################################################
 
 RELFLAGS := $(PLATFORM_RELFLAGS)