Minidiag: Enable minidiag by default

This CL enabled the pre-boot diagolostic tools entry in recovery mode.

BRANCH=none
BUG=b:176947486
TEST=CQ
TEST=FEATURES=test P2_TEST_FILTER='*' emerge-zork vboot_reference
TEST=manually build, flash FW to device, and confirm minidiag works

Signed-off-by: Meng-Huan Yu <menghuan@chromium.org>
Change-Id: I8500cbaaa4e8d0f1aa668d56d38095ecdcf50aaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2617391
Reviewed-by: Hung-Te Lin <hungte@chromium.org>