Move bootstage timer out of lib/time.c

The standalone example does not have get_timer() defined, so we cannot
rely on it being available.

Move the timer function into boootstage.c to avoid this problem.

This corrects a build breakage for the standalone example on some boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>

include/common.h

@@ -206,13 +206,6 @@ typedef void (interrupt_handler_t)(void *);
 
 #endif /* CONFIG_SERIAL_MULTI */
 
-/*
- * Return the time since boot in microseconds, This is needed for bootstage
- * and should be defined in CPU- or board-specific code. If undefined then
- * millisecond resolution will be used (the standard get_timer()).
- */
-ulong timer_get_boot_us(void);
-
 /*
  * General Purpose Utilities
  */