update_kernel: Add option to update kernel in the alternate boot slot

The kernel in the non-booting slot can be updated by using the --partition and
--rootfs options. However, this is a little tedious to discover what the
current kernel is and set the options appropriately. Add an option to do
this automatically, for lazy people like me.

TEST=Use --ab_update in combination with the "Chrome OS failsafe kernel
     flashing" doc to boot 5.10 on grunt.

Change-Id: I64da029bac56b9039c2d1c9f5f033209eb1e97a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2616592
Commit-Queue: Anand K Mistry <amistry@chromium.org>
Tested-by: Anand K Mistry <amistry@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
1 file changed