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>