Add constants for partition numbers

Initially only use these constants locally within disk_layout_util.sh

BUG=None
TEST=built and booted an image, ensured mounts were recognized

Change-Id: I329d4bdfb0e3ef4d41cb8a25f231946a80db792c
Reviewed-on: https://chromium-review.googlesource.com/349570
Commit-Ready: Steven 'Steve' Kendall <skendall@neverware.com>
Tested-by: Steven 'Steve' Kendall <skendall@neverware.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed