vb2: Modify phase2 behavior for S3 resume case

If a platform does verification of memory init then it must be careful
to use the same slot for resume that it booted from.  This is
accomplished by adding a context flag to indicate this is an S3 resume
and that vboot should treat it differently than a normal boot.

When this flag is set then the same slot that was booted is read from
VBNV and re-used for the resume path, without adjusting any try flags.
If this slot is B then the related context flag is set.

This will allow the firmware updater to update the other (non-booted)
slot and set flags indicating that on the next boot the updated slot
should be tried, while still allowing suspend/resume to work with the
existing firmware slot.

This assumes that the last tried slot was successfully booted, which
should be a safe assumption since the system was able to boot and then
suspend.  It isn't reliable to check last_fw_result for "success"
status because that status is only set some time after boot when
chromeos-setgoodkernel calls chromeos-firmwareupdate --mode=bootok
and so it may still report a status of "trying" on resume depending
on how soon after boot the suspend happened.

It also avoids setting the vboot flag indicating that a slot choice
was made in order to avoid altering the try counter on failure since
this is explicitly not attempting to boot the new slot.

BUG=chromium:577269
BRANCH=glados
TEST=manually tested on chell:
1) ensure that booting from slot A resumes from slot A.
2) ensure that booting from slot B resumes from slot B.
3) do RW update while booted from slot A (so the flags are set to try
slot B) and ensure that suspend/resume still functions properly using
current slot A.
4) do RW update while booted from slot B (so the flags are set to try
slot A) and ensure that suspend/resume still functions properly using
current slot B.

Change-Id: I500faef2b5d19a02f32839976354abf6d551c9f6
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Previous-Reviewed-on: https://chromium-review.googlesource.com/328812
(cherry picked from commit 19e70098fb0eafb61c8092781629630732c583e2)
Reviewed-on: https://chromium-review.googlesource.com/329008

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