sandbox: Support trace feature

Support tracing on sandbox by adding suitable CONFIG options. To enable it,
compile U-Boot with FTRACE=1.

The timer functions are marked to skip tracing, since these are called from
the tracing code itself, and we want to avoid an infinite loop.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/cpu.c b/arch/sandbox/cpu/cpu.c
index dd8d495..e9385de 100644
--- a/arch/sandbox/cpu/cpu.c
+++ b/arch/sandbox/cpu/cpu.c
@@ -37,7 +37,7 @@
 	os_usleep(usec);
 }
 
-unsigned long timer_get_us(void)
+unsigned long __attribute__((no_instrument_function)) timer_get_us(void)
 {
 	return os_get_nsec() / 1000;
 }