bootstage: Make use of BOOTSTAGE_ID_RUN_OS in show_boot_progress()

This changes the number 15 as used in boot_stage_progress() to use the
new name provided for it. This is a separate patch because it touches
so many files.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>