bootstage: Require timer_get_boot_us() to be defined

At present we provide a default version of this function for use by
bootstage. However it uses the system timer and therefore likely requires
driver model. This makes it impossible to time driver-model init.

Drop the function and require boards to provide their own. Add a sandbox
version also. There is a default implememtation in lib/time.c for boards
which use CONFIG_SYS_TIMER_COUNTER.

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

include/bootstage.h

@@ -2,6 +2,7 @@
  * This file implements recording of each stage of the boot process. It is
  * intended to implement timing of each stage, reporting this information
  * to the user and passing it to the OS for logging / further analysis.
+ * Note that it requires timer_get_boot_us() to be defined by the board
  *
  * Copyright (c) 2011 The Chromium OS Authors.
  *
@@ -209,7 +210,7 @@ enum bootstage_id {
 /*
  * 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()).
+ * you will get a link error.
  */
 ulong timer_get_boot_us(void);