firmware: Do not set recovery reason directly in LoadKernel()

LoadKernel() currently contains code that sets the recovery reason
directly (via direct nvdata access, bypassing the usual
VbSetRecoveryReason() helper) whenever it has a problem loading a
kernel. This seems to be an ancient vestige from the time when
LoadKernel() (and not VbSelectAndLoadKernel()) was still the external
API. In our current use, VbTryLoadKernel() will always immediately
override any recovery reason set this way.

This patch removes this pointless code to avoid confusion. Instead,
TryLoadKernel() is expanded to be able to tell the difference between
LoadKernel() return codes and set a more precise recovery reason based
on that.

BRANCH=None
BUG=chromium:692715
TEST=make runtests

Change-Id: Idd8bd6e16d5ef1472aa3b2b66468248726d5c889
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1859686
5 files changed