vboot: make dev boot target functions consistent

vb2_get_dev_boot_target() should only return:

  BOOT_USB iff vb2_dev_boot_usb_allowed()
  BOOT_LEGACY iff vb2_dev_boot_legacy_allowed()

Otherwise, fallback to BOOT_DISK.

BUG=None
TEST=make clean && make runtests
BRANCH=none

Change-Id: Ic67915d0888f640f6e7b04c91f99346412cc18ee
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2203301
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>
2 files changed