vboot/ui: Reorder UI errors

Reorder errors in enum vb2_ui_error to match the order in bmpblk. Also
rename VB2_UI_ERROR_EXTERNAL_BOOT_NOT_ENABLED to
VB2_UI_ERROR_EXTERNAL_BOOT_DISABLED for consistency with
VB2_UI_ERROR_ALTFW_DISABLED.

BUG=b:144969091
TEST=make runtests
TEST=emerge-asurada depthcharge
BRANCH=none

Cq-Depend: chromium:2682057
Change-Id: Id8859be3e451c077dd7689b51e98a410d5987874
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2746898
Reviewed-by: Joel Kitching <kitching@chromium.org>