Previously the bootloader unconditionally revoked unused secure boot key
digest slots while permanently enabling secure boot on the first boot,
ignoring CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS. Now the config is
honored on this path too: when set, the unused digest slots are left
un-revoked. This is safe as long as the debug and download interfaces are
disabled.
Update the Kconfig help and the Secure Boot v2 guide (en and zh_CN)
accordingly.