Use constants for partition numbers in mount_gpt_image.sh

BUG=None
TEST=built and booted an image, ensured mounts were recognized
CQ-DEPEND=I329d4bdfb0e3ef4d41cb8a25f231946a80db792c

Change-Id: I9a306479dd8c744806bf5a13f21b15e352f92a5a
Reviewed-on: https://chromium-review.googlesource.com/349580
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