vboot: Replace VB2_REQUEST_UI_CONTINUE with VB2_SUCCESS

In current codebase, VB2_REQUEST_UI_CONTINUE is for successful ui
function call, and VB2_SUCCESS is for breaking the main ui loop.

Replace most of the VB2_REQUEST_UI_CONTINUE with VB2_SUCCESS, and
replace those breaking requesting VB2_SUCCESS with VB2_REQUEST_UI_EXIT.
All the "VB2_REQUEST_UI_CONTINUE" and "VB2_REQUEST_UI_EXIT" are caught
in the ui loop. VB2_REQUEST_UI_CONTINUE does nothing while
VB2_REQUEST_UI_EXIT breaks the ui loop and return VB2_SUCCESS. Returning
VB2_SUCCESS and VB2_REQUEST_UI_CONTINUE to the ui loop now should do the
same thing, that is, continue process the ui.

VB2_REQUEST_UI_CONTINUE can be used to quickly return from ui functions
to the main UI loop, because it is not VB2_SUCCESS and makes VB2_TRY
return.

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

Signed-off-by: Chung-Sheng Wu <chungsheng@google.com>
Change-Id: If1b54657d09198196f9c646b6b907b4fd8faadce
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2714502
Tested-by: Chung-Sheng Wu <chungsheng@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Chung-Sheng Wu <chungsheng@chromium.org>
6 files changed