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>