chromeos-tpm-recovery: Clarify error message for boot mode
chromeos-tpm-recovery: Clarify error message for boot mode

chromeos-tpm-recovery only works when it's actually booted in recovery
mode, not when just running a recovery image in developer mode with
Ctrl+U. This distinction is often not super clear to non-firmware
people, so make the error message more explicit.

BRANCH=None
BUG=None
TEST=None

Change-Id: I18afa933f624f26d1f7949b9b586ab01daf7c801
Signed-off-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1913491
Reviewed-by: Curtis Malainey <9427d954bf16a65878c953cc565cfe9ab51a3885@chromium.org>
Reviewed-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot