Cache boot-fail-wait to avoid romfile access after POST.

Memory allocated with malloc_tmp() can't be used after the POST phase.
So, access boot-fail-wait in post phase and store it for the boot
phase to use.  This fixes the regression introduced by commit
59d6ca52.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Amos Kong <akong@redhat.com>
(cherry picked from commit 11a7234491cb2a027b0fa5e82af38a3e78b44c80)