bootstage: Eliminate when not enabled

When we do not have bootstage enabled, rather than include an empty
dummy function, we just don't reference it.  This saves us space in some
tight builds.  This also shows a few cases where show_boot_progress was
incorrectly guarded before.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>