vboot/ui: Refactor log_page api

Refactor code to use log_page_update to update log pages. Clean up
duplicated code. Split the cancel button logic to
log_page_show_back_or_cancel function.

BUG=b:174127808, b:157625765
TEST=make clean && CC=x86_64-pc-linux-gnu-clang make runtests
BRANCH=none

Cq-Depend: chromium:2641904
Signed-off-by: Chung-Sheng Wu <chungsheng@google.com>
Change-Id: Iaa8565c91c310e218dade45eb0efd56c72950512
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2641981
Reviewed-by: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Tested-by: Chung-Sheng Wu <chungsheng@chromium.org>
Commit-Queue: Chung-Sheng Wu <chungsheng@chromium.org>
2 files changed