vboot/ui: add screen stack functionality

Add a stack storing previous screen states.  When the user clicks
"Back" or presses ESC, revert to the previous state.

In order to deal with the possibility of a UI cycle (repeatedly
selecting the same sequence of screens) which would eventually
use up all available memory, re-use existing target screen states
within the stack.  In other words, when switching to a specific
screen which already exists in the stack, pop until that screen
is reached, rather than creating a duplicate stack entry.

BUG=b:146399181, b:158256196
TEST=make clean && make runtests

Change-Id: I6fbebc2abb11b26d95d4fcf841eb195b3d589396
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2214617
Reviewed-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Joel Kitching <kitching@chromium.org>
6 files changed