Use constants for partition numbers in update_kernel.sh

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

Change-Id: I2fc33197343981a73ff1a056ac595d1233d9abc4
Reviewed-on: https://chromium-review.googlesource.com/351240
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